The Sei Whale is found in waters all over the world, although it avoids tropical or polar regions. They are found in the Indian Ocean, Atlantic Ocean and Pacific Ocean.

They range in size from around 12 to 15 metres. The largest specimen ever reported was 20 metres in length. They weigh an average of 23 tonnes but weights of 27 tonnes have been reported. They are dark gray to black in colour. They have a sleek streamlined body, and a single ridge along the top of their head. Their underbelly is a lighter colour, and has been known to have a pinkish tinge. Randomly scattered patches of white on the whale are usually parasitic scars from copepods or other such sea creatures.

They feed on around 900 kilograms of small fish (and other aquatic creatures) per day. They do this by swimming through schools of fish, skimming the fish as they swim. They feed near the surface of the water, and when feeding they swim on their side. They are unable to sustain dives of more than 5 to 10 minutes at a time and cannot reach great depths as other whales can, however they make up for this with their speed and agility.