| 1 | SEK | Salaam Express Air Services | IATA: N8, IATA Prefix: 547, Ceased operations. | EAK | AVCODES |
| 2 | SEK | Société d'Exploration et de Constructions Aéronautiques (Groupe Aérospatiale) | Callsign: SECA, | F | AVCODES |
| 3 | SEK | Societe SECA | Callsign: SECA, | F | AVCODES |
| 4 | SEK | Star East Airline s.r.l. | IATA: 4R, Callsign: East Rider, | RO | AVCODES |
| 5 | SEK | Skyjet | | | ICAO1 |
| 6 | SEK | Hafen- und Bahnbetriebe der Stadt Krefeld | | D | TRAFFIC |
| 7 | SEK | Hafen- und Bahnbetriebe der Stadt Krefeld / Städtische Eisenbahn Krefeld | | DE | VKM |
| 8 | SEKAR | Sekar-Trans | | PL | TRAFFIC |
| 9 | SEKG | SPEER TRUCKING LLC | | | SCAC4 |
| 10 | SEKH | SHENZHEN ENERGY LOGISTICS COMPANY L | | | SCAC4 |
| 11 | SEKI | SEKIHYO LINE (PANAMA) SA | | | SCAC4 |
| 12 | SEKK | SEEKONK OIL CORPORATION | | | SCAC4 |
| 13 | SEKN | SPEEDY EAGLE TRUCKING INC | | | SCAC4 |
| 14 | SEKQ | SPICE TRUCKING INC | | | SCAC4 |
| 15 | SEKR | Southeast Kansas R.R. Co. | added 10/1988; 1/1994; eliminated 10/1996; 7/2000-7/2005 | | NAKINA |
| 16 | SEKR | Southeast Kansas R.R. Co. | added 10/88; 1/94; eliminated 10/96; 7/2000 | | NRHS |
| 17 | SEKR | SOUTHEAST KANSAS RAILROAD COMPANY | | | PWRC |
| 18 | SEKR | Southeast Kansas Railroad | | | RSRV |
| 19 | SEKR | SOUTHEAST KANSAS RAILROAD COMPANY | | | SCAC4 |
| 20 | SEKU | SEACO SRL | | BDS | BIC |
| 21 | SEKW | SEKO WORLDWIDE LLC | | | SCAC4 |
| 22 | SEKX | SEKISUI SPECIALTY CHEMICALS AMERICA | | | SCAC4 |
