A lap counter for use by a swimmer, has a case for attaching to the swimmer, and a compass sensor housed in the case for providing an output signal which changes between opposite directions along which the swimmer swims back and forth. An operating circuit includes a processor programmed to distinguish the change in the output signal of the compass sensor between the opposite directions to identify a reversal in direction of swimming by the swimmer and to count laps, each lap being based on two successive reversals in direction. The processor may also be programmed to distinguish the change in the compass sensor output signal to detect a rise above an upper threshold and a subsequent fall below a lower threshold, or vice versa, to identify, a wave in the output signal that represents a swimming stroke of the swimmer and to count the strokes.