Hyotissa hyotis specimens were collected at Espíritu Santo Island, Gulf of California, from March 1998 to March 1999. Water temperature was recorded, gut-content samples were obtained, phytoplankton abundance was estimated, and the reproductive status was inferred through the gonadic index. Gut content included phytoplankton, mostly diatoms (86.5%) and dinoflagellates (13.5%). The highest phytoplankton frequency in gut contents occurred from March to May, along with a growing phytoplankton abundance in seawater, matching the period of gonad development and the seasonal rise in temperature. Phytoplankton comprised diatoms (73.3%), dinoflagellates (10.4%), nannoplankton (18.8%), cyanobacteria (3.3%), and silicoflagellates (2.2%). The gonadic index suggests that this population has an annual reproductive pattern. Gonad development takes place from March to April, with the population ripening between April and July, matching the seasonal temperature rise from 22 °C to 27 °C, and spawning from July to September when temperatures reach a peak between 27 °C to 29 °C. From October onwards the population enters a postspawning and reproductive-inactivity phase.