| 3001 | ISCX | Imperial Sugar Co. | added 7/60 | | NRHS |
| 3002 | ISCX | International Sugar Co. | eliminated 10/60 | | NRHS |
| 3003 | ISCX | Interlake Steel Corp. | added 7/65 (reporting marks changed from IICX); 7/65-10/66; eliminated 10/67 | | NRHS |
| 3004 | ISCX | Interlake, Inc. | added 4/71; 1/75; eliminated 7/76 | | NRHS |
| 3005 | ISCX | ISC, Inc. | 1/94-7/2000 | | NRHS |
| 3006 | ISCX | *CARNEGIE-ILLINOIS STEEL CORP. [ILLINOIS STEEL CO. TANK LINE] | | | PWRC |
| 3007 | ISCX | *INTERNATIONAL SUGAR CO. [IMPERIAL SUGAR CO.] | | | PWRC |
| 3008 | ISCX | *INTERLAKE, INC. [INTERLAKE STEEL CORP.] | | | PWRC |
| 3009 | ISCX | +ISC, INC. | | | PWRC |
| 3010 | ISCX | Illinois Steel Company (ISC Inc.) | | | RSRV |
| 3011 | ISCX | ISC INC | | | SCAC4 |
| 3012 | ISD | ISD Avia Ltd. | Callsign: Isdavia, | UA | AVCODES |
| 3013 | ISD | Innovative Software Design | | ZA | AVCODES |
| 3014 | ISD | ISD Avia | | | ICAO1 |
| 3015 | ISDE | IRONSIDE ENTERPRISES INC | | | SCAC4 |
| 3016 | ISDO | INSIDEOUT ART | | | SCAC4 |
| 3017 | ISDU | ISRO DEPARTMENT OF SPACE GOVT OF INDIA | | IND | BIC |
| 3018 | ISE | Ise Electric Railway Co. | Owned by Kintetsu | J | TRAFFIC |
| 3019 | ISER | Ise Railway | 3. sector railway in Mie Prefecture | J | TRAFFIC |
| 3020 | ISES | ISLAND ENTERPRISES | | | SCAC4 |
| 3021 | ISEU | I C E SERVICE GROUP INC | | | SCAC4 |
| 3022 | ISF | International Stabilisation Assistance Force | Afghanistan Peace keeping forces | GB | AVCODES |
| 3023 | ISF | International Sabilisation Assistance Force | | | ICAO1 |
| 3024 | ISFD | ISF TRADING INC | | | SCAC4 |
| 3025 | ISFF | INTERSCAN SCHIFFAHRTSGESELLSCHAFT M | | | SCAC4 |
| 3026 | ISFN | ICICLE SEAFOODS INC | | | SCAC4 |
| 3027 | ISFR | ISAAC FREIGHT LIMITED | | | SCAC4 |
| 3028 | ISFX | International Sugar Feed Co. | 8/1920-7/1923 | | NAKINA |
| 3029 | ISFX | INTERNATIONAL SUGAR FEED CO. | | | PWRC |
| 3030 | ISFZ | SEACASTLE CHASSIS | | | SCAC4 |
| 3031 | ISG | Sixcargo S.p.A. | IATA: 6P, Callsign: Italcargo, Name changed to Clubair Sixgo S.p.A. | I | AVCODES |
| 3032 | ISG | Clubair Sixgo S.p.A. | IATA: 6P, Callsign: Italcargo, Name changed from Sixcargo S.p.A. | I | AVCODES |
| 3033 | ISG | Club Air | IATA: 6P | | ICAO1 |
| 3034 | ISG | ISG RAILWAYS | | | PWRC |
| 3035 | ISG | ISG Railways | | | RSRV |
| 3036 | ISGE | ISG EXPRESS INC | | | SCAC4 |
| 3037 | ISGI | INTERSTATE SERVICE GROUP INC | | | SCAC4 |
| 3038 | ISGL | INTERMODAL SOLUTIONS GROUP LLC | | | SCAC4 |
| 3039 | ISGT | ISG TRANSPORTATION INC | | | SCAC4 |
| 3040 | ISGU | INCHCAPE SHIPPING SERVICE | | | SCAC4 |
| 3041 | ISGX | International Steel Group d/b/a ISG Burns Harbor, Inc. | 7/2005 (not listed) | | NAKINA |
| 3042 | ISGX | INTERNATIONAL STEEL GROUP d/b/a ISG BURNS HARBOR, INC. | | | PWRC |
| 3043 | ISGX | INTERNATIONAL STEEL GROUP D/B/A ISG | | | SCAC4 |
| 3044 | ISH | Island Winds, Inc. | Callsign: Island Hop, | USA | AVCODES |
| 3045 | ISHI | INTER SHIP INC | | | SCAC4 |
| 3046 | ISHN | INKEL, STEPHANE INC | | | SCAC4 |
| 3047 | ISHU | ISLAND SHIPPING LINE LTD. | | | SCAC4 |
| 3048 | ISHX | INTERSTATE SHREDDING LLC | | | SCAC4 |
| 3049 | ISI | Sky Line | Callsign: Irina, | GE | AVCODES |
| 3050 | ISI | Island Air | | | ICAO1 |
| 3051 | ISIA | INLINE SUPPLY INC | | | SCAC4 |
| 3052 | ISIE | ISI EXPRESS (N Y) INC | | | SCAC4 |
| 3053 | ISIG | INDUSTRIAL SEATING INCORPORATED | | | SCAC4 |
| 3054 | ISII | INDUSTRIAL STITCHTECH INC | | | SCAC4 |
| 3055 | ISIL | I S I LIVESTOCK | | | SCAC4 |
| 3056 | ISIN | INDUSTRIAL SALES INC | | | SCAC4 |
| 3057 | ISIP | ISI LOGISTICS SOUTH INC | | | SCAC4 |
| 3058 | ISIQ | ISI | | | SCAC4 |
| 3059 | ISIU | ISI AUTOMOTIVE AUSTRIA GMBH | | A | BIC |
| 3060 | ISIV | ISI | | | SCAC4 |
| 3061 | ISIZ | INTERMODAL SERVICES, INC. | | | PWRC |
| 3062 | ISIZ | Intermodal Services | | | RSRV |
| 3063 | ISIZ | INTERMODAL SERVICES, INC. | | | SCAC4 |
| 3064 | ISK | Intersky Luftfahrt GmbH | IATA: 3L, Callsign: Intersky, IATA Prefix: 576, Designator 3L recalled by IATA | A | AVCODES |
| 3065 | ISK | Intersky Luftfahrt GmbH | IATA: 3L, Callsign: Intersky, Filed for bankruptcy and ceased operations on 09 November 2015. Designator 3L reinstated by IATA | A | AVCODES |
| 3066 | ISK | Intersky Luftfahrt | | | ICAO1 |
| 3067 | ISKG | INTERSEE SCHIFFAHRTSGESELLSCHAFT MB | | | SCAC4 |
| 3068 | ISKL | IST TRUCKING LTD | | | SCAC4 |
| 3069 | ISL | Eagle Air Ltd. (Arnarflug H/F) | Callsign: Eagleair, Designator VL recalled by IATA | IS | AVCODES |
| 3070 | ISL | Eagle Air Ltd. (Arnarflug H/F) | IATA: VL, Callsign: Eagleair, Designator VL recalled by IATA | IS | AVCODES |
| 3071 | ISL | Landsflug ehf | IATA: X9, Name changed to City Star Airlines ehf | IS | AVCODES |
| 3072 | ISL | City Star Airlines ehf | IATA: X9, Callsign: Islandia, Ceased operation on 30 January 2008. Designator X9 re-instated by IATA. Name changed from Landsflug ehf | IS | AVCODES |
| 3073 | ISL | Flugfélag Austurlands | Callsign: Eastland, | IS | AVCODES |
| 3074 | ISL | Landsflug | | | ICAO1 |
| 3075 | ISL | Island Line Ltd. | Real: IL, 1999: 13.4km standard gauge, electrified at 660V= 3rd rail | GB | TRAFFIC |
| 3076 | ISL | Infraserv Logistics GmbH | | DE | VKM |
| 3077 | ISLA | ISLAND AIR SERVICE | | | SCAC4 |
| 3078 | ISLC | ISLAND CARGO CONSOLIDATORS, INC. | | | SCAC1 |
| 3079 | ISLC | ISOLAB INC | | | SCAC4 |
| 3080 | ISLD | ISLAND TOWING LTD | | | SCAC4 |
| 3081 | ISLE | ISLE LOADEM | | | SCAC4 |
| 3082 | ISLJ | INTEGRAL SOLUTIONS LLC | | | SCAC4 |
| 3083 | ISLL | ISLAND TRANSPORT LTD | | | SCAC4 |
| 3084 | ISLS | ISLAND SHIPPING LTD | | | SCAC4 |
| 3085 | ISLT | ISLAND TUG & BARGE CO | | | SCAC4 |
| 3086 | ISLU | INDUSTRIAL SOLVENTS & CHEMICALS PVT, LTD | | IND | BIC |
| 3087 | ISLU | *ISLANDIA LIMITED | | | PWRC |
| 3088 | ISLU | *RETIRED 04-06-93 ATP | | | PWRC |
| 3089 | ISLU | Islandia | | | RSRV |
| 3090 | ISLU | INDUSTRIAL SOLVENTS & CHEMICALS PVT | | | SCAC4 |
| 3091 | ISLV | ISLAND ENVIRONMENTAL SERVICES INC | | | SCAC4 |
| 3092 | ISLY | INTERSPEC SYSTEMS LIMITED | | | SCAC4 |
| 3093 | ISM | AUO Aeroclub Aist-M | Callsign: Stork, | RUS | AVCODES |
| 3094 | ISM | Auo Airclub AIST-M | | | ICAO1 |
| 3095 | ISMA | INTERNATIONAL SHIP MANAGEMENT AND A | | | SCAC4 |
| 3096 | ISMI | INSTA-MIX | | | SCAC4 |
| 3097 | ISMO | INDUSTRIAL SOLUTIONS MANAGEMENT INC | | | SCAC4 |
| 3098 | ISMS | ISLANDER MARITIME S A | | | SCAC4 |
| 3099 | ISMU | INTERNATIONAL SOLUTIONS MANAGEMENT, LLC | | USA | BIC |
| 3100 | ISMU | INTERMODAL STRUCTURES | | | SCAC4 |
