Abstract 1. 1. Pinealectomy of the white sucker, Catostomus commersoni, had significant effects on the free-running circadian rhythm of locomotor activity. With fish kept under constant darkness pinealectomy resulted in a significant alteration in period length and decrease in the precision of circadian activity. 2. 2. In a number of cases circadian activity was split into two components that free-ran independently. 3. 3. The pineal organ is suggested to function in the coupling of a multi-oscillator system making up circadian organization in this fish.