Evidence for sexual niche partitioning in Mercurialis perennis and Silene dioica in Wales and Trophis involucrata in Costa Rica is presented. The differences in M. perennis and T. involucrata are spatial in nature with males of M. perennis being found more frequently in patches of high soil pH while males of T. involucrata are found more frequently in patches of low total phosphorous content. Sexual niche partitioning in S. dioica was found to be temporal in nature, with females being precocious in their dominance of the leaf canopy. Sexual dimorphisms and the results of intersexual competition trials in S. dioica are discussed. Two alternative hypotheses for the evolution of sexual niche partitioning are discussed; the first of these suggests that niche differences are the result of selection against deleterious intersexual competition, and the second suggests that selection for interspecific competitive ability may lead to niche differences and may possibly be implicated in the evolution of dioecism fro...