The occurrence of seasonality in bullfrog (Lithobates catesbeianus) spermatogenesis was analyzed by quantifying germ cell populations throughout the year, which were divided into bimesters, from November 2001 to October 2002. Body weight, liver weight, fat body weight and gonadal weight were taken and bimonthly grouped. Using these data, the gonadosomatic, hepatosomatic, and liposomatic indexes were calculated. All animals were kept in the same cement cages in a frog farm, and sampled every month. The cages were under the influence of climate variables changes, such as photoperiod, temperature, rainfall and relative humidity. Body weight was higher during Spring and Summer, and a significant reduction has occurred between July and August. The highest gonadosomatic index values were registered from May to October, coinciding with the period of gonadal preparation for reproduction. Also, the reproduction of bullfrog is directly related to environmental factors, mainly due to Summer and Winter humidity variations.