Pygmy mice were first brought to the attention of biologists in 1887 when Oldfield Thomas described a diminutive species of cricetine rodent, _Hesperomys_ (_Vesperimus_) _taylori_. The description was based on a specimen obtained by William Taylor from San Diego, Duval County, Texas. C. Hart Merriam (1892:70) described _Sitomys musculus_ on the basis of specimens from Colima [City of], Colima, Mexico. Merriam (_loc. cit._) mentioned that the two kinds of mice, _Hesperomys taylori_ and _Sitomys musculus_, "in general appearance look almost precisely like the common house mouse (_Mus musculus_) but are still smaller and have shorter tails." He placed the two species in the genus _Sitomys_. Frederick W. True in 1894 regarded them as composing a distinct subgenus of _Sitomys, Baiomys_. According to True (1894:758), _S. taylori_ and _S. musculus_ possessed a different combination of characters (ascending ramus of mandible short and erect, condyle terminal, coronoid process well-developed, uncinate, and near the condyle, size small, tail short, plantar tubercles six, soles hairy) than either _Vesperimus_, or _Onychomys_ (which had been considered as a subgenus of _Hesperomys_ until 1889). In 1907, E. A. Mearns accorded _Baiomys_ generic rank. Osgood (1909:252) treated _Baiomys_ us a subgenus of _Peromyscus_, whereas, Miller, in 1912, regarded _Baiomys_ as a distinct genus. Most recent students of North American mammals have followed Miller, but usually with reservations. Ellerman (1941:402) emphasized that the taxonomic position of the genus was uncertain, and wrote that _Baiomys_ "... seems to be considerably distinct from _Peromyscus_, and may perhaps be a northern representative of _Hesperomys_ or one of the small South American genera." Only two comprehensive analyses of geographic variation and interspecific taxonomic relationships have been made; the first was by Osgood (1909) who had fewer than a fourth of the specimens of _Baiomys_ available to me; the second was by Hooper (1952a:90-97) who contributed importantly to understanding the relationships of the two living species in central Mexico. No attempts heretofore have been made to correlate and understand the relationships of the five fossil species to one another and to the living species assigned to the genus. Six objectives of the following report are to: (1) list characters taxonomically useful in recognizing species and subspecies; (2) record amount of variation within and between populations; (3) correlate observed variations with known biological principles; (4) show geographic ranges of the two living species; (5) indicate relationships between fossil and living species of the genus; and (6) clarify the systematic position of the genus. MATERIALS, METHODS AND ACKNOWLEDGMENTS