High‐speed motion pictures have enabled us to study the swimming movements of Bosmina longirostris. These small cladocerans swim using antennal movements which superficially resemble those of a human breaststroke. The time taken for each stroke is relatively constant, and the movements of the antennae during a stroke are nearly invariant. Thus, the animals vary their average speed mainly by varying the interval between strokes.In the absence of antennal forces, a Bosmina body would move broadside. Forces produced by the antennae of a swimming Bosmina constrain the body to move edgewise. The shape of the body is not the shape which minimizes drag. The orientation and shape of the body may reflect opposing selection for reducing drag and the ability of predators to find Bosmina and for the ability of predators to manipulate Bosmina.