When male gulf killifish Fundulus grandis were netted at a consistent time each day for 2–3 weeks, gonad weight and fat stores increased or decreased depending on the time of day the handling occurred. The phases, amplitudes, and patterns of these response rhythms may be influenced by day length and season. In spring, gonad responses were similar at temperatures of 20 and 28 C. In fall, when gulf killifish were placed on a 48-hour diel cycle (12 hours light, 36 hours darkness), fat-response rhythms recycled every 20 hours (for example, similar responses occurred at 0, 20, and 40 hours after the onset of light). Thus, daily photoperiod appears to entrain a circadian rhythm of responsiveness to handling similar to its entrainment of a circadian responsiveness to light.