The circadian behavioral rhythm in a population ofParamecium multimicronucleatumwas examined by means of a computer/video system. A parameter, “traverse frequency”;, was introduced to index a locomotor activity of the specimens. The frequency is the average number of paramecia images per hour which individually traversed underneath photo‐cells placed on the CRT‐screen of the TV monitor. The traverse frequency was entrained and phase‐shifted by light‐dark (LD) cycles. It was highest at about 4 h after the beginning of the dark period and lowest at about 4 h after the beginning of the light period in LD 12:12 (20°C). Its rhythm free‐ran in DD and LL with about a 24 h period, the LL rhythm being relatively less stable than the DD rhythm. The free‐running period of DD rhythm was temperature‐compensated for 15–25°C. The circadian rhythmicity in the traverse frequency implies that the daily locomotor behavior ofParameciumis controlled by its circadian pacemaker.