A single median ridge on the head extends from the blowholes to near the tip of the rostrum in the sei whales. The dorsal fin is tall and hooked compared to other baleen whales, and is forward on the body compared to both fin and blue whales. The flippers are small.
Body colour is usually blue-grey but lighter underneath and the area of the throat grooves is white.
Some white baleen plates may occur beneath the rostrum while the remainder are black with a fine white fringe. Because the sei whale feeds near the surface, the dorsal fin shows for a longer period than in other baleen whales which feed well below the surface. Sei whales are similar to Bryde's whale and can be easily confused at sea if the three distinct ridges on the head of the Bryde's whale cannot be seen. After protection of blue whales and the depletion of fin whale populations, sei whales formed the major part of the Antarctic whale catch in the mid-1960's. There have been no authenticated strandings of sei whales in Australia.