One-allele isolating mechanisms should make the evolution of reproductive isolation between potentially hybridizing taxa easier than two-allele mechanisms, but the generality of one-allele mechanisms in nature has yet to be established. A potentially important one-allele mechanism is sexual imprinting, where the mate preferences of individuals are based on the phenotype of their parents. Here I test the possibility that sexual imprinting promotes reproductive isolation using sympatric species of threespine sticklebacks (Gasterosteus aculeatus). Sympatric species of sticklebacks consist of large benthic species and small limnetic species that are reproductively isolated and adapted to feeding in different environments. I fostered families of F₁ hybrids between the species to males of both species. Preferences of these fostered females for males of either type revealed little or no effect of sexual imprinting on assortative mating. However, F₁ females showed preferences for males that were similar to themselves in length, suggesting that size-assortative mating may be more important than sexual imprinting for promoting reproductive isolation between species pairs of threespine sticklebacks.