The distribution and abundance of the endangered Baja California killifish, Fundulus lima, and its interaction with exotic fishes were studied in oases of the central part of Baja California peninsula, Mexico (Río San Ignacio and Río La Purísima basins), from October 2002 to July 2004. Five fish species were recorded: the native killifish and 4 exotics (Cyprinus carpio, Poecilia reticulata, Xiphophorus hellerii, and Tilapia cf. zillii). Two new populations of the killifish are reported for the Río San Ignacio as well as confirmation of occurrence in the upper Río La Purísima. Density of the killifish in the Río San Ignacio showed a significant inverse correlation with that of the exotic redbelly tilapia (Tilapia cf. zillii). Baja California killifish densities in both rivers decreased significantly after the hurricane-induced flood event of 23 September 2003, but exhibited notable recovery 10 months later. Relative abundance of killifish at the type locality, San Ignacio oasis, decreased significantly during an 8-year period after introduction of tilapia. The status of Baja California killifish should be changed from threatened to endangered to reflect continued reductions in distribution as result of proliferation of nonnative fishes, particularly redbelly tilapia. Management efforts to reduce abundance of exotic fishes are needed to ensure persistence of the endemic Baja California killifish.