THE zebrafish (Danio rerio) may be useful for mutational analyses of vertebrate circadian clock mechanisms if efficient assays of circadian rhythmicity are available. Using an automated video image analysis system, we found robust circadian rhythms in the locomotor activity of larval (10- to 15-day-old) zebrafish maintained in constant conditions. Activity was rhythmic in > 95% of the animals tested. The timing of peak activity in constant conditions was determined by the prior light:dark cycle, with highest activity during the subjective day. The mean freerunning period of the activity rhythms was 25.6 h, and the within-experiment standard deviation of freerunning period ranged from 0.5 to 1.0 h. Therefore, it should be possible to detect mutations that lengthen or shorten the freerunning circadian period of zebrafish activity rhythms by 1–2 h.