Life-history parameters (growth, reproduction, and survival) were measured from one genetically heterogeneous and three homogeneous populations of tadpole shrimp (Triops longicaudatus) raised under three competitive levels: five, ten, and 16 shrimp per 38 liter aquarium. Comparisons were made between the homogeneous populations (three monomorphic selfing lines) and a heterogeneous population (a mixture of the three lines) to test for increased productivity in genetically variable populations, a prediction of the tangled bank hypothesis for a short-term advantage to sexual reproduction. In most comparisons, the mixture of inbred lines performed better than expected from their individual performances in pure cultures but did not outperform the best performing line in any comparison. Decomposition of the mixed culture's performance into performances of the component inbred lines showed that no line experienced release from competition in the mixed culture. No genotype X environment interaction was found for any of the three fitness correlates, indicating that all three lines performed similarly under the three competitive conditions. These results provide little support for the tangled bank hypothesis and suggest that the levels of heterogeneity necessary for detectable resource partitioning in these shrimp may involve factors not considered in this study.