The rate of occurrence of click trains of two harbor porpoises (Phocoena phocoena) were counted during 14 nights. We developed an echolocation signal detection system that was harnessed to a porpoise and activated a light when the animal emitted an echolocation signal. This device, referred to as a click-light, detects echolocation signals above 150 dB re 1 μPa in the 28–180 kHz range. Echolocation rates, i.e., occurrences of click trains, changed frequently, ranging from 0 to 25 per minute. Echolocation rates were affected by feeding, individual difference, and enclosure type such as the net enclosure and the pool. The porpoise echolocation rates seemed to show acclimation.