Native mammals of 38 species are known from 35 of the islands that surround the Baja California Peninsula of western Mexico (D. J. Hafner, pers. comm.). The majority of these (67 populations of 24 species of 11 species-groups) are rodents, and among the most common insular rodent of these islands is the deer mouse, Peromyscus maniculatus, which is known from 10 islands. Huey (1964) and Lindsay (1983) summarized the history of mammal surveys on the islands surrounding the Peninsula. The most extensive surveys were conducted by Nelson (1921) and Burt (1932). With few exceptions, survey efforts subsequent to 1960 targeted specific taxa, islands, or small sets of islands (e.g., Ryckman and Ryckman, 1963; Banks, 1964a, 1964b, 1967; Brand and Ryckman, 1969; Schultz et al., 1970; Lawlor, 1971). At the same time, impact of humans on these islands has increased at an exponential rate during the past half-century (Bahre, 1983). Adverse effects of introduced non-native animals on native plant and animal communities of islands have been well documented (e.g., Owen, 1977; Konecny, 1987a, 1987b; Van Rensburg and Bester, 1988). Three endemic species of woodrats, Neotoma anthonyi (Isla Todos Santos), N. martinensis (Isla San Martin), and N. bunkeri (Isla Coronados), are regarded as extinct due to predation by introduced feral house cats (Felis silvestris) and depletion of food resources (Schultz et al., 1970; Smith et al., 1993; E. Mellink, in litt.). Three other endemic insular taxa (Lepus insularis of Isla Espiritu Santo, Sylvilagus mansuetus of Isla San Jose, and Odocoileus hemionus cerrosensis of Isla Cedros) are considered rare and potentially threatened (Groombridge, 1993). Further, 0. J. Reichman (pers. comm.) has observed behavioral differences in populations of pocket mice (Chaetodipus spinatus) on islands in the Gulf of California lacking predators that would leave these mice more susceptible to introduced