In experimental pools open to emigration, mosquitofish (Gambusia affinis) resided at higher densities than topminnow (Poeciliopsis occidentalis). When Gambusia were introduced to Poeciliopsis pools, all topminnow maintained residency. Only 11% of the Gambusia emigrated from the mixed populations before the plant cover was removed; with no cover, 59% left. A significant number of Gambusia in single species pools also responded to a loss of cover by emigrating. Continuing residency of Poeciliopsis after introduction of Gambusia supports findings that displacement by mosquitofish is not an immediate process. Frayed fins on resident topminnow suggest short-term agonistic interactions. Coexistence of an exotic species (Gambusia) and an endangered desert fish (Poeciliopsis) may depend on the complexity, or spatial variation, of the Southwest's few remaining cienegas. The opportunity to disperse from pools in these habitats may be an important factor in Poeciliopsis survival; such emigratory behavior can be tested in open experimental systems.