In some animals sex is determined after conception by environmental factors (environmental sex determination. ESD). In the amphipodGammarus duebenisex is reportedly determined by photoperiod: there is a higher proportion of males in broods reared under long‐day than under short‐day photoperiods. It has been proposed that this is an adaptive response to seasonal population dynamics. A test of the hypothesis would be to demonstrate changes in the degree to which sex is determined by the environment in populations from different latitudes with different dynamics. This study reports such a test. Environmental response is significantly less strong in a southern population with a long breeding season than in a northern one in which breeding is seasonally restricted. Moreover, the threshold of the ESD cue for male or female determination is not defined when the breeding season is weakly seasonal. There is a broad correlation between latitude (and hence breeding seasonality) and the strength of ESD response across a series of population studies. Similarities between theGammarussystem of sex determination and that of the Atlantic silversideMenidia menidia,a fish with thermal ESD, are discus