| 301 | UCIR | UNION COUNTY INDUSTRIAL RAILROAD CO | | | SCAC4 |
| 302 | UCIS | UNIQUE CARRIERS INC | | | SCAC4 |
| 303 | UCIX | Unichema Chemicals, Inc. | added 1/1986; 1/1994; eliminated 4/1996 | | NAKINA |
| 304 | UCIX | Unichema Chemicals, Inc. | added 1/86; 1/94; eliminated 4/96 | | NRHS |
| 305 | UCIX | UNICHEMA CHEMICALS INC | | | PWRC |
| 306 | UCK | State Firefighting Service Of The Eastern Kazakhstan Region | Callsign: Galeta, | KZ | AVCODES |
| 307 | UCK | Air Division of the Eastern Kazakhstan Region | | | ICAO1 |
| 308 | UCKX | Union Coal & Coke Co. (Bethlehem Steel Co.) | 8/1920-7/1923 | | NAKINA |
| 309 | UCKX | BETHLEHEM STEEL CO. [UNION COAL & COKE CO.] | | | PWRC |
| 310 | UCLD | UNVIERSAL CARLOADING & DISTRIBUTING CO | | | PWRC |
| 311 | UCLD | US CARGO LINK LLC | | | SCAC4 |
| 312 | UCLI | UNITCARGO CONTAINER LINE INC | | | SCAC4 |
| 313 | UCLL | UNITED COASTAL LOGISTICS LTD | | | SCAC4 |
| 314 | UCLQ | UNITED COMMODITIES LOGISTICS LLC | | | SCAC4 |
| 315 | UCLR | UNITED CARTAGE LLC | | | SCAC4 |
| 316 | UCLS | UNIVERSAL CARRIERS LLC | | | SCAC4 |
| 317 | UCLSK | UniCredit Leasing Slovakia, a.s. | | SK | VKM |
| 318 | UCLU | UNICORN LINES (PTY) LTD. | | | PWRC |
| 319 | UCLU | Unicorn Lines | | | RSRV |
| 320 | UCLU | UNICORN LINES (PTY) LTD | | | SCAC4 |
| 321 | UCLX | Union Chemical Carline, Inc. | added 7/1956; 10/1957-4/1970; eliminated 11/1973 | | NAKINA |
| 322 | UCLX | Vulcan Materials Co., Chemicals Division | added 11/1973; 1/1975-7/2005; eliminated 10/2005 | | NAKINA |
| 323 | UCLX | Basic Chemicals Co., LLC | added 1/2006; eliminated 4/2007 | | NAKINA |
| 324 | UCLX | Occidental Chemicals Corp. | added 4/2007 | | NAKINA |
| 325 | UCLX | Union Chemical Carline, Inc. | added 7/56; 10/57-4/70 | | NRHS |
| 326 | UCLX | Vulcan Materials Co., Chemicals Division | 1/75-7/2000 | | NRHS |
| 327 | UCLX | OCCIDENTAL CHEMICALS CORP. [BASIC CHEMICALS CO., LLC; VULCAN MATERIALS CO. CHEMICALS DIV.; UNION CHEMICAL CARLINE, INC.] | | | PWRC |
| 328 | UCLX | Vulcan Minerals | | | RSRV |
| 329 | UCLX | BASIC CHEMICALS COMPANY LLC | | | SCAC4 |
| 330 | UCM | University of Central Missouri | Callsign: Mule Flight, | USA | AVCODES |
| 331 | UCM | UNION CARBIDE (WEST VIRGINIA) | | | PWRC |
| 332 | UCMI | UNITED CARGO MANAGEMENT INC | | | SCAC4 |
| 333 | UCMR | UCM RESITA SA | | RO | VKM |
| 334 | UCMX | United Commercial Co., Inc. | 7/1930-4/1938 | | NAKINA |
| 335 | UCMX | United Clays, Inc. | added 7/1990; 1/1994-7/2000; eliminated 1/2002; 7/2005 (not listed) | | NAKINA |
| 336 | UCMX | United Commercial Co., Inc. | Apr-38 | | NRHS |
| 337 | UCMX | United Clays, Inc. | added 7/90; 1/94-7/2000 | | NRHS |
| 338 | UCMX | UNITED CLAYS, INC. | | | PWRC |
| 339 | UCMX | United Clays | | | RSRV |
| 340 | UCNN | U S CONNECTION INC | | | SCAC4 |
| 341 | UCNY | UNITRANS CONSOLIDATED INC | | | SCAC4 |
| 342 | UCO | Linea Aerea del Cobre Ltda. (LADECO) | IATA: UC, Callsign: Ladeco, | RCH | AVCODES |
| 343 | UCO | Ucoaviacion | | | ICAO1 |
| 344 | UCOI | UNITED CENTRAL ORCHIDS INC | | | SCAC4 |
| 345 | UCOP | UNITED COOPERATIVES INC | | | SCAC4 |
| 346 | UCOX | Union Carbide Corp., Chemicals Division | added 7/1964; 7/1965-10/1966; eliminated 10/1967 | | NAKINA |
| 347 | UCOX | Union Carbide Corp., Chemicals and Plastics | added 10/1967; eliminated 1/1968 | | NAKINA |
| 348 | UCOX | Union Carbide Corp., Chemicals and Plastics Operations Division | added 1/1968; 4/1970-1/1975; eliminated 1/1979 | | NAKINA |
| 349 | UCOX | Union Carbide Corp., Chemicals and Plastics Division | added 1/1979; 1/1980; eliminated 7/1980 | | NAKINA |
| 350 | UCOX | Union Carbide Corp. | added 7/1980; eliminated 10/1980 | | NAKINA |
| 351 | UCOX | Union Carbide Corp., Chemicals and Plastics Division | added 10/1980; eliminated 5/1983 | | NAKINA |
| 352 | UCOX | Union Carbide Corp. | added 5/1983 | | NAKINA |
| 353 | UCOX | Union Carbide Corp., Chemicals and Plastics Co., Inc. | eliminated 1/1993 | | NAKINA |
| 354 | UCOX | Praxair, Inc. | added 1/1993; 1/1994; eliminated 4/1996; 7/2000 (not listed); 7/2005 | | NAKINA |
| 355 | UCOX | Union Carbide Corp., Chemicals Division | added 7/64; 7/65-10/66; eliminated 10/67 | | NRHS |
| 356 | UCOX | Union Carbide Corp., Chemicals and Plastics | added 10/67; eliminated 1/68 | | NRHS |
| 357 | UCOX | Union Carbide Corp., Chemicals and Plastics Operations Division | added 1/68; 4/70-1/75; eliminated 1/79 | | NRHS |
| 358 | UCOX | Union Carbide Corp., Chemicals and Plastics Division | added 1/79; 1/80; eliminated 7/80 | | NRHS |
| 359 | UCOX | Union Carbide Corp. | added 7/80; eliminated 10/80 | | NRHS |
| 360 | UCOX | Union Carbide Corp., Chemicals and Plastics Division | added 10/80; eliminated 5/83 | | NRHS |
| 361 | UCOX | Union Carbide Corp. | added 5/83 | | NRHS |
| 362 | UCOX | Union Carbide Corp., Chemicals and Plastics Co., Inc. | eliminated 1/93 | | NRHS |
| 363 | UCOX | Praxair, Inc. | added 1/93; 1/94; eliminated 4/96; 7/2000 | | NRHS |
| 364 | UCOX | PRAXAIR, INC. [UNION CARBIDE CORP.] | | | PWRC |
| 365 | UCOX | Union Carbide Corporation | | | RSRV |
| 366 | UCOX | PRAXAIR INC | | | SCAC4 |
| 367 | UCPX | Union Carbide Chemicals Co., Division of Union Carbide Corp. | added 7/1961; eliminated 10/1963 | | NAKINA |
| 368 | UCPX | Union Carbide Corp., Chemicals Division | added 10/1963; 7/1965-10/1966; eliminated 10/1967 | | NAKINA |
| 369 | UCPX | Union Carbide Corp., Chemicals and Plastics | added 10/1967; eliminated 1/1968 | | NAKINA |
| 370 | UCPX | Union Carbide Corp., Chemicals and Plastics Operations Division | added 1/1968; eliminated 4/1969 | | NAKINA |
| 371 | UCPX | Essem Corp. | added 4/1969; 4/1970-1/1980; eliminated 7/1985 | | NAKINA |
| 372 | UCPX | Union Carbide Corp., Chemicals Division | 7/65-10/66; eliminated 10/67 | | NRHS |
| 373 | UCPX | Union Carbide Corp., Chemicals and Plastics | added 10/67; eliminated 1/68 | | NRHS |
| 374 | UCPX | Union Carbide Corp., Chemicals and Plastics Operations Division | added 1/68; eliminated 4/69 | | NRHS |
| 375 | UCPX | Essem Corp. | added 4/69; 4/70-1/80 | | NRHS |
| 376 | UCPX | ESSEM CORP. - UNION CARBIDE CORP., LESSEE | | | PWRC |
| 377 | UCPX | Essem Corporation | | | RSRV |
| 378 | UCR | Aero-Charter Ukraine Ltd. | {Callsign: Charter Ukraine, Name changed to Joint Stock Company "Air Company Aero-Charter"};SRCKEY=AVCODES | UA | |
| 379 | UCR | Joint Stock Company "Air Company Aero-Charter" | IATA: DW, Callsign: Charter Ukraine, IATA Prefix: 187, Founded in 1997. Ceased operations. Name changed from Aero-Charter Ukraine Ltd. | UA | AVCODES |
| 380 | UCR | Aero-Charter Ukraine | IATA: DW | | ICAO1 |
| 381 | UCR | Utah Coal Route | 8/1920-1/1980 | | NAKINA |
| 382 | UCR | Utah Coal Route | 4/38-1/80 | | NRHS |
| 383 | UCR | UTAH COAL ROUTE | | | PWRC |
| 384 | UCR | Utah Coal Route | | | RSRV |
| 385 | UCRI | USA CARRIERS, INC | | | SCAC1 |
| 386 | UCRN | UNICORN TRANSPORTATION INC | | | SCAC4 |
| 387 | UCRS | UPPER CANADA RAILWAY SOCIETY | | | PWRC |
| 388 | UCRS | UPPER CANADA RAILWAY SOCIETY | | | SCAC4 |
| 389 | UCRT | USA CARTAGE INC | | | SCAC4 |
| 390 | UCRY | Utah Central Ry. Co. | added 1/1996; eliminated 4/1997; 7/2000-7/2005 | | NAKINA |
| 391 | UCRY | Utah Central Ry. Co. | added 1/96; eliminated 4/97; 7/2000 | | NRHS |
| 392 | UCRY | UTAH CENTRAL RAILWAY CO. | | | PWRC |
| 393 | UCRY | Utah Central Railroad | | | RSRV |
| 394 | UCRY | UTAH CENTRAL RAILWAY COMPANY | | | SCAC4 |
| 395 | UCS | United Carriers Systems, Inc. | Callsign: United Carriers, | USA | AVCODES |
| 396 | UCS | United Carriers Systems | | | ICAO1 |
| 397 | UCSB | UNILOG CARGO SA | | | SCAC4 |
| 398 | UCSN | UNITED CARGO SERVICES INC | | | SCAC4 |
| 399 | UCSS | UNIBRIK CUSTOM SERVICES LTD | | | SCAC4 |
| 400 | UCSU | UNIVERSAL CONTAINER SERVICES LIMITED | | GB | BIC |
