In The Log of the Sea of Cortez, that memorable treatise of science, adventure, and philosophy, John Steinbeck (1951) made bare mention of mammals. Of course, the main purpose of that effort was to chronicle a trip to the Gulf of California to collect invertebrates in the company of Steinbeck’s friend and scientist, Ed Ricketts. The party visited four islands—Tiburón, Coronados, San José, and Espíritu Santo. At anchor off Isla Tiburόn, Steinbeck reported a swarm of bats that approached their boat. One bat was collected but, to the best of our knowledge, it was never identified or preserved. Aside from some descriptions of taxa (e.g., Butt 1932), relatively little was known at the time about mammals from islands in the Sea of Cortés. There is now a reasonably rich history of systematic and biogeographic studies of mammals in and adjacent to the Sea of Cortés (for general reviews, see Orr 1960; Huey 1964; Lawlor 1983; and Hafner and Riddle 1997). Here we summarize much of that information and explore biogeographic patterns that emerge from it, add important recent records of bats, and evaluate new evidence about the origins of insular faunas and the ecological processes and human impacts that affect colonization and persistence of mammals on gulf islands. The terrestrial mammalian fauna of islands in the Sea of Cortés (including islands off the Pacific coast of Baja California) comprises 45 species, of which 18 currently are recognized as endemics (but see below), representing 5 orders, 9 families, and 14 genera (app. 12.1). Collectively they share relationships with mainland representatives on both sides of the gulf and are divisible into 28 clades of species or species groups (app. 12.2). Rodents are disproportionately represented, constituting a total of 35 species and 76 of 97 total insular occurrences, and they are the only nonvolant mammals to become established on distant oceanic islands. In addition, except for the few species of lagomorphs, which occur only on landbridge islands, a greater proportion of mainland species of rodents occurs on islands than is the case for other groups of mammals.