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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 (Rio San Ignacio and Rio La Purisima 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 Titapia cf. zillii). Two new populations of the killifish are reported for the Rio San Ignacio as well as confirmation of occurrence in the upper Rio La Purisima. Density of the killifish in the Rio 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.