The length of time required to reject scale allografts was examined in groups of male gulf killifish (Fundulus grandis) treated daily with either 4-h thermoperiods (daily intervals of 30°C during a 20°C continuum), single meal feedings, or net-chasing disturbances at one of six different times after the onset of daily photoperiods (LD 12:12 or LD 14:10). Scale allograft survival varied by 30–60% in each timed stimulus experiment as a function of the time of day when the stimulus was provided. The phase relationship between two circadian neuroendocrine oscillations, previously proposed to regulate physiologic and behavioral conditions in gulf killifish, may have important influences on the immune system as well. The phase of one oscillation is thought to be set by the daily photoperiod, and the other is preferentially set by the nonphotic daily stimulus. © 1995 Wiley-Liss, Inc.