In the fish genus Poeciliopsis (Poeciliidae) all female clones coexist in Mexican desert streams with males and females of closely related sexually reproducing species. Past work has confirmed that genetically fixed differences in the use of food resources and habitat space contribute to the coexistence of sexual and asexual lines. The proposed study consists of two parts. (1) Competition experiments within and between two clonal lines and a sexual line will be conducted in artificial streams to test whether genetically variable populations show lowered intraspecific competition relative to monoclonal populations. Results from this study will shed light on the ecological advantage of sexual reproduction. (2) Field studies will quantify natural densities and relative frequencies of the two clonal lines and the sexual line. Results from these collections will show the relative reproductive success of the above three lines in a range of densities and relative frequencies. Such ecological studies are necessary for increasing knowledge of the costs and benefits of genetic variability in natural populations. However, this study also has potential applied value as well. Genetically uniform populations can grow well in controlled environments, such as well fertilized and well watered agricultural fields, but are unlikely to perform as well in uncontrolled environments. Both aquaculturists and conservationists are confronted with maintaining populations in uncontrolled environments, and therefore need to understand the ecological benefits of maintaining genetic variation in these populations.