Despite the enormous volume of research concerning the various effects of chemicals with endocrine-disrupting properties in fish, there is still very little evidence that endocrine disruption can adversely affect individual fertility and, hence, pose problems for the population. In the present study, guppies (Poecilia reticulata) were fed with the anti-androgenic fungicide vinclozolin at concentrations ranging from 1.8 to 180 mg/kg from 8-14 wk of age. Male sperm count and the intensity of his sexual display behavior were significantly reduced in treatment groups, which was in line with the results of previous studies. Here, we show further that these impairments translate into reduced fertility, measured as the size of the female's first clutch. Also, this reduced fertility was correlated to the male sperm count, but not to the intensity of the male sexual display. Finally, by crossing exposed with unexposed animals, we show that the adverse effect of vinclozolin on reproduction is mediated through the male alone.