Responses of the ovary of the catfish, Heteropneustes fossilis, to various regimes of photoperiod and temperature, were studied during the different periods of the annual ovarian cycle. During the preparatory and post-spawning periods moderate temperatures of 25 degrees C and above, regardless of photoperiod, were more favorable for the formation of yolky (stage III) oocytes than coller temperatures (below 23 degrees C). Even under conditions of continuous dark or light, ovarian development occurred as a function of warm temperatures. Gravid ovaries were maintained beyond the spawning period only with a temperature of 30 degrees C, photoperiod notwithstanding. Nevertheless, moderate or cool temperatures did not prevent the ovarian regression that occurred by the first week of October in the post-spawning period. Regulation of ovarian activity at least in part by an endogenous circannual rhythm appears possible.