The ecologically highly important effect of intermale competition is assessed here in a new approach to evaluating endocrine disruption at the population level. A guppy (Poecilia reticulata) male exposed to the antiandrogenic DDT metabolite p,p'-DDE competed against an unexposed male for the opportunity to fertilize a female. Offspring were then assigned to sires using microsatellites in genetic paternity analysis. In addition, a suite of male sexual characteristics including sperm count, coloration, and sexual behavior were also measured. p,p'-DDE produced no significant harmful effect on either the male sexual characteristics or the success in siring young, although the highest sublethal dose tested was only a factor 10 below the dose producing 100% mortality. At present, data on the antiandrogenic potency of p,p'-DDE are highly ambiguous. This study supports the increasing amount of evidence that the demasculinizing action of this chemical is often weak and highly variable across populations, even within the same species.