The reproductive cycle of Hyotissa hyotis at Isla La Ballena, Gulf of California, was studied from August 1995 to December 1996. The histologic analysis revealed that spawning in H. hyotis occurred as a continued phenomenon throughout virtually the whole period of study, with a steep rise in the spawning effort during the warmest season in July and August. Optimal temperature for peak spawning was between 27°C and 28.5°C; however, spawning organisms were found at a temperature as low as 20.5°C. A significant correlation between spawning and temperature was observed. The sex ratio of adult organisms did not significantly differ from 1M:1F. Gonad-Index (GI) values are indicative of gonad development, hence these may be regarded as a reliable indicator of the time of spawning. Maximum GI values coincided with the highest percentage or organisms in the maturity stage. No significant relationship was detected between GI and temperature.