| 2901 | IRT | Irtysh Avia | IATA: IT, Callsign: Irtysh, Designator IT reinstated by IATA | KZ | AVCODES |
| 2902 | IRT | Interot Airways | IATA: IQ, Callsign: Interot, IATA Prefix: 614, Name changed to Augsbury-Airways GmbH. Founded in 1980. | D | AVCODES |
| 2903 | IRT | Irtysh-Avia | IATA: IT, Callsign: Irtysh, Designator IT recalled by IATA | KZ | AVCODES |
| 2904 | IRT | Irtysh-Avia | Callsign: Irtysh, Designator IT recalled by IATA | KZ | AVCODES |
| 2905 | IRT | Toos Airlines | Callsign: Toos Air, | IR | AVCODES |
| 2906 | IRT | Irtysh Airlines | IATA: IT | | ICAO1 |
| 2907 | IRT | Interborough Rapid Transit | City: New York | USA | TRAFFIC |
| 2908 | IRTD | INDIAN RIVER TRANSPORT LTD | | | SCAC4 |
| 2909 | IRTF | IRVIN TRANSFER | | | SCAC1 |
| 2910 | IRTF | I & T TRANSFER | | | SCAC4 |
| 2911 | IRTM | INTEGRATED RECYCLING TECHNOLOGIES I | | | SCAC4 |
| 2912 | IRTN | INTER RAIL TRANSPORT LIMITED | | | SCAC4 |
| 2913 | IRTR | INTERMOUNTAIN RIGGING & | | | SCAC4 |
| 2914 | IRTS | INGERSOLL-RAND TRANSPORTATION SERVICES COMPANY | | | SCAC1 |
| 2915 | IRTS | INGERSOLL-RAND TRANSPORTATION SERVI | | | SCAC4 |
| 2916 | IRTT | European Rail Transport Feroviar | | RO | TRAFFIC |
| 2917 | IRTU | ISOTANK SERVICES LIMITED | | | SCAC4 |
| 2918 | IRTX | INTER-RAIL TRANSPORT | | | PWRC |
| 2919 | IRTX | Inter-Rail Transport | | | RSRV |
| 2920 | IRTX | INTER-RAIL LINK | | | SCAC4 |
| 2921 | IRU | Chabahar Airline | Callsign: Chabahar, | IR | AVCODES |
| 2922 | IRU | Chabahar Airlines | | | ICAO1 |
| 2923 | IRUN | Imperial Russian Navy | Real: IRN | RUS | TRAFFIC |
| 2924 | IRV | Safat Airlines | Callsign: Safat Air, | IR | AVCODES |
| 2925 | IRV | Safat Airlines | | | ICAO1 |
| 2926 | IRVCB | IRV CARANSEBES SA | | RO | VKM |
| 2927 | IRVG | IRVING ENERGY DISTRIBUTION AND MARK | | | SCAC4 |
| 2928 | IRVI | IRVING OIL TERMINALS INCORPORATED | | | SCAC4 |
| 2929 | IRVT | IRVIN, DICK INC | | | SCAC4 |
| 2930 | IRW | Aram Airline | Callsign: Aram, | IR | AVCODES |
| 2931 | IRW | Aram Airline | | | ICAO1 |
| 2932 | IRWD | IRONWOOD TRADING CO | | | SCAC4 |
| 2933 | IRX | Aria Tour | Callsign: Aria, | IR | AVCODES |
| 2934 | IRX | Aria Tour | | | ICAO1 |
| 2935 | IRX | Wasatch Oil Co., Idaho Division | 1/1950-7/1954 | | NAKINA |
| 2936 | IRX | Idaho Refining Co. | 1/1941-4/1947 | | NAKINA |
| 2937 | IRX | Independent Refining Co. | 8/1920-7/1923 | | NAKINA |
| 2938 | IRX | Wasatch Oil Co., Idaho Division | Jan-52 | | NRHS |
| 2939 | IRX | *INDEPENDENT REFINING CO. | | | PWRC |
| 2940 | IRX | *WASATCH OIL CO. [IDAHO REFINING CO.] | | | PWRC |
| 2941 | IRXL | IRMAR TRANSPORT LLC | | | SCAC4 |
| 2942 | IRY | Ilam Airline | IATA: YE, Callsign: Ilam, Name changed from Eram Airlines | IR | AVCODES |
| 2943 | IRY | Eram Airlines | Callsign: Eram, Name changed to Ilam Airline | IR | AVCODES |
| 2944 | IRY | Eram Air | | | ICAO1 |
| 2945 | IRYM | ILLINOIS RAILWAY MUSEUM | | | PWRC |
| 2946 | IRZ | Saha Airlines Services | Callsign: Saha, | IR | AVCODES |
| 2947 | IRZ | Saha Airlines Services | | | ICAO1 |
| 2948 | IS | Island Airlines, Inc. | ICAO: ISA, Callsign: Island, | USA | AVCODES |
| 2949 | IS | IDS Aircraft Ltd. d/b/a I.D.S. Heathrow Exective Jets | ICAO: IDS, Callsign: Fanjet, Name changed from IDS Aircraft Ltd. | GB | AVCODES |
| 2950 | IS | Intercontinental Pacific Airways, Inc. | ICAO: IPC, Callsign: Interpac, IATA designator changed to IO | RP | AVCODES |
| 2951 | IS | IDS Aircraft Ltd. | ICAO: IDS, Callsign: IDS, Name changed to IDS Aircraft Ltd. d/b/a I.D.S. Heathrow Exective Jets | GB | AVCODES |
| 2952 | IS | AIS Airlines B.V. | ICAO: PNX, Callsign: Spinner, | NL | AVCODES |
| 2953 | IS | Sepehran Airlines | ICAO: SHI, Callsign: Shirazi, | IR | AVCODES |
| 2954 | ISA | Island Airlines, Inc. | IATA: IS, Callsign: Island, | USA | AVCODES |
| 2955 | ISA | Island Air | Callsign: Island, | USA | AVCODES |
| 2956 | ISAA | ISAACS MOVING & STORAGE | | | SCAC4 |
| 2957 | ISAE | ISAAC, LEON | | | SCAC4 |
| 2958 | ISAF | ISLAND SUPERIOR AIR FILTERS | | | SCAC4 |
| 2959 | ISAF | ISAF S.A. | | RO | TRAFFIC |
| 2960 | ISAF | ISAF SA | | RO | VKM |
| 2961 | ISAH | IONICS MASS SPECTROMETRY GROUP INC | | | SCAC4 |
| 2962 | ISAM | ISAM S.r.l. | | IT | VKM |
| 2963 | ISAN | INLAND STATE AIR ENTERPRISES INC | | | SCAC4 |
| 2964 | ISAP | INTERNATIONAL SEA & AIR SHIPPING | | | SCAC4 |
| 2965 | ISAR | ISARIC TRANSPORT INC | | | SCAC4 |
| 2966 | ISAR | Isarperle | | D | TRAFFIC |
| 2967 | ISAU | NORDIC BULKERS AB | | | SCAC4 |
| 2968 | ISAW | INLACO SAIGON | | | SCAC4 |
| 2969 | ISAY | INTERNATIONAL SHIPPING AND LOGISTIC | | | SCAC4 |
| 2970 | ISB | Caraibes Air Transport | IATA: WS, Callsign: Island Bird, Designator WS recalled by IATA | GP | AVCODES |
| 2971 | ISBC | INTEQUIP MINING SALES & SERVICES LT | | | SCAC4 |
| 2972 | ISBL | ISABELLA SHIPOWNER S A | | | SCAC4 |
| 2973 | ISBU | SEABOARD SYSTEM RAILROAD | | | SCAC4 |
| 2974 | ISC | Island Air Charters, Inc. | Callsign: Barracuda, | USA | AVCODES |
| 2975 | ISC | Interporto Servizio Cargo | | I | TRAFFIC |
| 2976 | ISC | ISC-INTERPORTO Servizi Cargo S.P.A. | | IT | VKM |
| 2977 | ISCB | ISABELLA SHIPPING COMPANY | | | SCAC1 |
| 2978 | ISCB | ISABELLA SHIPPING COMPANY LTD BERMU | | | SCAC4 |
| 2979 | ISCC | INTERNATIONAL SHIPPING COMPANY INC. | | | SCAC4 |
| 2980 | ISCF | INTERNATIONAL STONE CONCEPTS | | | SCAC4 |
| 2981 | ISCG | INDUSTRIAL SERVICES CONTRACTORS INC | | | SCAC4 |
| 2982 | ISCI | INTERNATIONAL TRANSPORT EXPRESS COR | | | SCAC4 |
| 2983 | ISCL | INDIA STEAMSHIP (A DIV OF | | | SCAC4 |
| 2984 | ISCO | INTERMODAL SALES CORPORATION | | | SCAC4 |
| 2985 | ISCP | INSTRON CORP | | | SCAC4 |
| 2986 | ISCQ | INTEGRITY SHIPPING CORP | | | SCAC4 |
| 2987 | ISCR | INLAND SEAFOOD CORP | | | SCAC4 |
| 2988 | ISCT | INTRINSIC TRANSPORT INC | | | SCAC4 |
| 2989 | ISCU | STAR CONTAINER SPAIN S.A. | | E | BIC |
| 2990 | ISCU | I.S.C. CHEMICALS LTD. | | | PWRC |
| 2991 | ISCU | I.S.C. Chemicals | | | RSRV |
| 2992 | ISCX | ISC, Inc. | 1/1994-7/2005 | | NAKINA |
| 2993 | ISCX | Interlake, Inc. | added 4/1971; 1/1975; eliminated 7/1976 | | NAKINA |
| 2994 | ISCX | Interlake Steel Corp. | added 7/1965 (reporting marks changed from IICX); 7/1965-10/1966; eliminated 10/1967 | | NAKINA |
| 2995 | ISCX | International Sugar Co. | eliminated 10/1960 | | NAKINA |
| 2996 | ISCX | Imperial Sugar Co. | added 7/1960 | | NAKINA |
| 2997 | ISCX | Carnegie-Illinois Steel Corp. | 7/1937-4/1938 | | NAKINA |
| 2998 | ISCX | Illinois Steel Co. | 7/1935 | | NAKINA |
| 2999 | ISCX | Illinois Steel Co. Tank Line | 8/1920-7/1932 | | NAKINA |
| 3000 | ISCX | Illinois Steel Co. (Carnegie-Illinois Steel Corp.) | Apr-38 | | NRHS |
