| 1 | UI | Eurocypria Airlines Ltd. | ICAO: ECA, Callsign: Eurocypria, IATA Prefix: 475, Liquidated on 29 November 2010 after EU failed to approve merger with Cyprus Airways | CY | AVCODES |
| 2 | UI | Alaska Seaplane Service, L.L.C. | IATA Designator changed to J5 | USA | AVCODES |
| 3 | UI | Société de Transport Aerien du Rwanda (STAR) | Callsign: Star, | RWA | AVCODES |
| 4 | UI | Cool Tours d/b/a San Juan Aviation | IATA Designator changed to JI | PRI | AVCODES |
| 5 | UI | Auric Air Services Limited | ICAO: AUK, Callsign: Auric Services, | EAT | AVCODES |
| 6 | UI | Ultimate Jetcharters, Inc. | ICAO: UJC, Callsign: Ultimate, IATA designator changed to UE. | USA | AVCODES |
| 7 | UIA | UNI Airways Corporation | IATA: B7, Callsign: Glory, IATA Prefix: 525, | RC | AVCODES |
| 8 | UIA | Uni Air | | RC | TRAFFIC |
| 9 | UIAW | UIA WORLDWIDE LOGISTICS INC | | | SCAC4 |
| 10 | UIBU | UNION INTERNATIONAL BITUMEN FACTORY | | | SCAC4 |
| 11 | UIC | Utah Idaho Central R.R. Co. | 7/1923-7/1936 | | NAKINA |
| 12 | UIC | Utah Idaho Central R.R. | 4/1938-7/1954 | | NAKINA |
| 13 | UIC | Utah Idaho Central R.R. | 4/38-1/52 | | NRHS |
| 14 | UIC | *UTAH IDAHO CENTRAL RAILROAD CO. | | | PWRC |
| 15 | UIC | *MARCHEM LTD | | | PWRC |
| 16 | UICC | UNITED INDUSTRIAL SERVICES | | | SCAC4 |
| 17 | UICO | UNITED BRIDGE TRANSPORTATION LLC | | | SCAC4 |
| 18 | UICQ | UNDERWOOD INDUSTRIAL CONTRACTORS IN | | | SCAC4 |
| 19 | UIDS | UNITED STATIONERS SUPPLY CO | | | SCAC4 |
| 20 | UIET | UNITED TRANSPORT LLC | | | SCAC4 |
| 21 | UIFU | UNITEX INTERNATIONAL FORWARDING (HK) LTD | | HK | BIC |
| 22 | UIID | UNICORN INDUSTRIES INC | | | SCAC4 |
| 23 | UIIN | US INTERMODAL INC | | | SCAC4 |
| 24 | UILC | UNITED INTERMODAL LLC | | | SCAC4 |
| 25 | UIME | U & I MOVERS EXPRESS | | | SCAC4 |
| 26 | UIMW | UNITED INTERNATIONAL MANAGEMENT LTD | | | SCAC4 |
| 27 | UIMX | United Iron & Metal LLC | 7/2005 (not listed) | | NAKINA |
| 28 | UIMX | UNITED IRON & METAL LLC | | | PWRC |
| 29 | UIMX | UNITED IRON & METAL LLC | | | SCAC4 |
| 30 | UINC | UNICOVER INC | | | SCAC4 |
| 31 | UINL | UNITRANS INTERNATIONAL INC | | | SCAC4 |
| 32 | UINR | UNITRANS INC | | | SCAC4 |
| 33 | UIQC | UNIVERSAL CARGO MANAGEMENT INC | | | SCAC4 |
| 34 | UIR | Ukrair, Aviacompany Ltd. | Callsign: Ukrair, Ceased operations. | UA | AVCODES |
| 35 | UISI | UNITED INDUSTRIAL SUPPLY (1985) INC | | | SCAC4 |
| 36 | UISN | UIS TRANSPORTATION INC | | | SCAC4 |
| 37 | UISZ | USCO UNDUSTRY SASKATHEWAN | | | PWRC |
| 38 | UIT | Universitetet I Tromso | Callsign: Spirit, ICAO Callsign changed to Artic. | N | AVCODES |
| 39 | UIT | Universitetet I Tromso | Callsign: Artic, ICAO Callsign changed from Spirit. | N | AVCODES |
| 40 | UITA | UNITED INTERMODAL TRANSPORT INC | | | SCAC4 |
| 41 | UITK | UNIFIED INTERMODAL TRANSPORT INC | | | SCAC4 |
| 42 | UITT | U.S. TRANS TECH, INC | | | SCAC1 |
| 43 | UITT | US TRANS TECH | | | SCAC4 |
| 44 | UIVL | UMUGISHA INVESTMENT LLC | | | SCAC4 |
| 45 | UIVQ | UNIVERSITY MOVING & STORAGE | | | SCAC4 |
| 46 | UIWX | Union Iron Works, subsidiary of Riley Stoker Corp. | added 1/1968; eliminated 4/1970 | | NAKINA |
| 47 | UIWX | Riley Stoker Corp. | added 4/1970; 4/1970-1/1994; eliminated 7/1998 | | NAKINA |
| 48 | UIWX | Union Iron Works, subsidiary of Riley Stoker Corp. | added 1/68; eliminated 4/70 | | NRHS |
| 49 | UIWX | Riley Stoker Corp. | added 4/70; 4/70-1/94; eliminated 7/98 | | NRHS |
| 50 | UIWX | RILEY STOKER CORP. [UNION IRON WORKS] | | | PWRC |
| 51 | UIWX | Riley Stoker Corporation | | | RSRV |
| 52 | UIZ | ULEN INDUSTRIES | | | PWRC |
