Clonal and sexual coexistence is common in a number of vertebrate taxa, even though the "cost of sex" makes such coexistence theoretically unlikely. The Frozen Niche-Variation (FNV) model explains this coexistence on the basis of differences in overall niche breadth and competition between clones and sexuals. In the present study I examined two predictions of the FNV model. First, I examined the prediction that genetically variable populations have higher relative fitness when compared with monoclonal populations by comparing the performances of clonal and outcrossed sexual strains of Poeciliopsis in monocultures at two densities. The prediction of increased overall productivity for the sexuals was verified, with net reproductive rates for the sexuals being between 2 and 4 times as high as the clones. Second, I tested the prediction that derived clones will successfully compete with their sexual progenitor(s) in the narrow range to which the clones are adapted, while the sexuals should coexist because of their ability to use a wider range of resources than any single clone. I examined this prediction by comparing performance variables (eg., growth, fecundity, and survival) of each strain in pure culture with their partitioned performance from the mixed treatments. Clonal performance increased in mixtures compared to monocultures, as expected. However, the expectation that the sexual's performance would be less affected by mixtures than the clones' performance, was not met. The sexuals had reduced growth and fecundity on par with the increase in both variables in the clones. Therefore, support for the FNV model was mixed. Although the performance in monocultures suggests that the sexuals have a wider niche-breadth than the clones, performances in mixtures do not indicate such a relationship. Switching of behaviors or resource use patterns between mixed and pure cultures may have caused the equivocal results.