Drawing on work with passerine birds, investigations on the self‐selection of photoperiod in a constant environment by pumpkinseed sunfish(Lepomis gibbosus)capable of controlling their lighting have been carried out. Each aquarium with lighting equipment was set up in a separate, light‐tight compartment in a quiet, constant‐temperature room. Under one treatment, an overhead fluorescent light could be turned on and off by breaking a photobeam. Under two further treatments, breaking the photobeam turned the light on for three minutes. Fish were fed daily at a regular hour. The latter two treatments, but not the first, resulted in data containing diurnal rhythmic components as analyzed by autocorrelation techniques. The results are discussed in the context of different methods of measuring “internal clock” mechanisms.