Blue whales are carnivores, although they only eat very tiny things, no bigger than your pinky finger. Their favorite food is krill, or shrimp-like euphausiids, that are up to three inches long. Blue whales must eat two to four tons of krill a day during the feeding season to survive the rest of the year. They concentrate on feeding during the polar summers primarily around the Channel Islands, Monterey Bay, and the Farallon Islands/Cordell Bank. During the winter months, they migrate to the warmer waters in Mexico and Costa Rica.

The blue whale usually feeds at depths of less than 100 m (330').

Blue whales feed almost exclusively on shrimplike crustaceans known as "krill". A blue whale may consume up to 5.5 - 6.4 metric tons (6 - 7 tons) of food per day during the summer feeding season. That's 7,715 pounds of krill (small shrimp-like organisms) per day! During the other 8 months of the year, it apparently doesn't eat anything, living off of stored fat.

The blue whale has long, flexible throat grooves. It feeds by using these groves to expand its throat and drawing in water laden with prey, then forcing the water out through its baleen plates. These plates filter out the prey organisms, which the whale then swallows.