Tamiasciurus mearnsi (Townsend, 1897) is a small-sized tree squirrel commonly called Mearns's squirrel. One of 3 species that comprise the genus Tamiasciurus, T. mearnsi, is endemic to the Sierra de San Pedro Mártir in Baja California, Mexico; it is considered rare and occurs only at low densities. T. mearnsi is federally listed as “Threatened” in Mexico and is also included in the International Union for the Conservation of Nature and Natural Resources Red List of Threatened Species as “Endangered.” The introduction of the eastern gray squirrel (Sciurus carolinensis) may be a serious threat to this relict species.