I have noted, as Blair (_loc. cit._) did, that _B. taylori_ utters high-pitched squeals in a "singing" posture resembling that of the coyote, yet remains silent when being handled. The northern pygmy mouse makes runways in the grass, in miniature resembling those of _Microtus_, and often uses runways constructed by _Sigmodon_. A small firm nest of finely shredded plant material (mostly grasses) is constructed in burrows or under logs, rocks, or fallen cactus plants. Thomas (1888:447) recorded nests of fine curly grass and cornsilk. Secondary refuge nests are not uncommon. Thomas (_loc. cit._) states, "If other mice live in the same place, the individuals of _Baiomys_ watch till others disappear, then suddenly steal part of the other nest and run to their own with it." _Enemies and food._--Little is recorded of the animals that prey upon the northern pygmy mouse. Twente and Baker (1951:120) found remains of _B. taylori_ in 16 per cent of barn owl pellets (_Tyto alba pratincola_) collected 21 mi. SW Guadalajara, Jalisco. Presumably most of the crepuscular and early nocturnal raptorial birds and carnivorous mammals feed on these mice. Food of _B. taylori_ consists in part of grass seeds and leaves, prickly pear (_Opuntia_ sp.) and the softer exposed parts of roots of vegetation among which the mice reside. _Reproduction._--The northern pygmy mouse breeds throughout the year. The only months in which I have not recorded pregnant females or females with young are June and October. Forty-one records of embryos or young per litter average 2.48 (less than in _B. musculus_), and range from as few as one to as many as four per litter. =Baiomys taylori allex= (Osgood) _Peromyscus allex_ Osgood, Proc. Biol. Soc. Washington, 17:76-77, March 21, 1904; Elliot, Field Columb. Mus. Publ., 105(6):135, July 1, 1905; Lyon and Osgood, Bull. U. S. Nat. Mus., 62:124, January 15, 1909. _Baiomys taylori allex_, Packard, Proc. Biol. Soc. Washington, 71:17, April 11, 1958; Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part). [_Peromyscus_] _allex_, Elliot, Field Columb. Mus. Publ., 95(4):175, July 15, 1904. _Peromyscus taylori paulus_, Osgood, N. Amer. Fauna, 28:255, April 17, 1909 (part). _Baiomys taylori paulus_, Miller, Bull. U. S. Nat. Mus., 79:137, December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:317, April 29, 1924 (part); Ellerman, The Families and Genera of Living Rodents, British Mus. Nat. Hist., 2:402, March 21, 1941 (part); Poole and Schantz, Bull. U. S. Nat. Mus., 178:259, March 6, 1942; Goldman, Smith. Miscl. Coll., 115:373, July 31, 1951 (part); Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3, 1955 (part); Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part).