The locomotor activity rhythms were examined by using an actograph with infra-red photo-electric switches for two species of wrasses, (Halichoeres tenuispinnis and Pteragogus flagellifera) under various light conditions. In H. tenuispinnis, the locomotor activity of almost all fish under light-dark cycle regimen (LD12: 12; 06: 00-18: 00 light, 18: 00-06: 00 dark) commenced somewhat earlier than the beginning of light period and continued till somewhat earlier than the beginning of the dark period. This species clearly showed freerunning activity rhythms under both constant illumination (LL) and constant darkness (DD). Therefore, H. tenuispinnis appeared to have a circadian rhythm. The length of the circadian period ranged from 23hr. 30min. to 23hr. 44min. under LL, and was from 23 hr. 39min. to 24hr. 18min. under DD. On the other hand, the locomotor activity of P. flagellifera occurred mostly in the light period under LD 12: 12. The activity of this species continued through LL, but was greatly suppressed in DD, so that none of the fish had any activity rhythm in both constant conditions. It was known from field observations that H. tenuispinnis burrowed and lay in sandy bottoms, while P. flagellifera hid and rested in bases of seagrasses and shallow crevices of rocks during the night. In the present two wrasses, it seemed that the above-mentioned difference of noctural behaviorwas closely related to the intensity of the endogenous factor in the activity rhythm.