Predictions of the Red Queen (RQ) model for the maintenance of sexual reproduction were examined using life-history data on clonal and sexual fish in the genus Poeciliopsis. Size and fecundity data were analyzed from fish from seven pools, and these measures were combined with parasitism intensity to address the assumptions and predictions of the RQ model. Significant differences in parasitism intensity were revealed among strains. One clone had greater parasitic load than either of the other strains in five of the seven pools examined. This difference in parasitism was independent of clonal frequency, in contrast to the RQ's assumption that the most common clone should be the most heavily parasitised. Also, parasitism intensity was uncorrelated with size-corrected fecundity in all three strains, indicating that differences in strain performance across pools was not influenced by parasitism. The combined evidence from current and previous studies indicate that the maintenance of sex in this clonal/sexual complex is most likely independent of the coevolutionary processes predicted by the RQ model, and is best explained by niche partitioning among strains.