The sei whale looks very similar to other large rorquals, especially its smaller relative the Bryde's whale. The best way to distinguish between it and Bryde's whale, apart from differences in baleen plates, is by the presence of lateral ridges on the dorsal surface of the Bryde's whale's rostrum. Large individuals can be confused with fin whales, unless the fin whale's asymmetrical head coloration is clearly seen. The fin whale's lower jaw's right side is white, and the left side is grey. When viewed from the side, the rostrum appears slightly arched (accentuated at the tip), while fin and Bryde's whales have relatively flat rostrums.