The influence of temperature on the relative reproductive rates of male and female sand gobies (Pomatoschistus minutus) was measured in aquarium experiments, with naturally changing as well as constant temperatures over the entire reproductive season. In this fish species the male builds a nest and cares for the eggs until hatching. Reproductive rate increased with increasing water temperature in both sexes, but more strongly among males. This reflects a difference in developmental rates between eggs while still unfertilized in the female ovary and after fertilization in the care of the male, respectively. Under constant temperature, there was still a seasonal effect on male reproductive rate, with incubation time decreasing over the season. Calculations based on egg densities and sizes of natural nests showed that males were able to care for the clutches of two average females simultaneously. Combining information about the differential effects of temperature on the reproductive rates of males and females, and on the egg-receiving capacity of the nests provided by the males, a male bias in the operational sex ratio is predicted to arise and increase over the season.