The blue whale is the largest mammal, possibly the largest animal, to ever inhabit the earth. Its body is long, somewhat tapered, and streamlined, with the head making up less than one-fourth of its total body length. Its rostrum (upper part of the head) is very broad and flat and almost U-shaped, with a single ridge that extends just forward of the blowholes to the tip of the snout. Its blowholes are contained in a large, rose “splash guard”, and the blow is tall and straight and over 20 feet (6 meters) high. Its body is smooth and relatively free of parasites, but a few barnacles attach themselves to the edge of the fluke and occasionally to the tips of the flippers and to the dorsal fin. There are 55-68 ventral grooves or pleats extending from the lower jaw to near the navel.

The blue whale is blue-gray in color, but often with lighter gray mottling on a darker background (or with darker spots on a lighter background). The underside of its flippers may be a lighter color or white, while the ventral (underside) of the fluke is dark. The blue whale acquires microorganisms called diatoms in the cold waters of the Antarctic and north pacific and north Antarctic which give the underside of its body a yellowish green caste. Because of this yellow color, the early whalers gave it the name “sulfur bottom”.

The largest creature ever known to have existed on Earth is the present-day blue whale . The largest dinosaur attained a length of about 22 m and weighed about 36 tons. Today's blue whale is even larger than its ancestors, and may reach a total length of 33 m and a weight of 145 tons. This giant animal is placid and shy. On the ocean surface, its normal cruising speed is about 12 knots, but it is capable of attaining 20 knots in short bursts. The maximum reported depth reached by the species is 194 fathoms. It is capable of remaining submerged beneath the surface for 50 minutes, although 10 to 15 minutes is more typical. The life span of a blue whale is about 30 years.

The calves measure 7 m at birth and weigh about 2 tons. By the time they are a year old, the youngsters measure 18 m.