_Diagnosis._--General ground color of upper parts between Wood Brown and Buffy Brown (all capitalized color terms are those of Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912), dorsal parts of fore- and hind-feet, and ankles white; in region of median venter, throat, and chin, hairs white to base; in lateral regions hairs Neutral Gray at base; dorsal hairs below tips Avellaneous, Neutral Gray at base; guard hairs black-tipped; tail white below, brownish above; nasals truncate anteriorly; frontalparietal suture forming an obtuse angle with median-parietal suture; alveolar-length of upper molar tooth-row and tail long. _Comparisons._--From _Baiomys musculus nigrescens_ (paratypes, from the Valley of Comitán, Chiapas, México), found to the north, _B. m. handleyi_ differs in: color paler dorsally and ventrally; fore- and hind-feet whitish instead of dusky to sooty; hairs in region of facial vibrissae white instead of brown; tail bicolored instead of unicolored; anterior tips of nasals square, not rounded; frontoparietal suture forming obtuse angle with median parietal suture instead of a right angle; tail and alveolar length of upper molar tooth-row significantly larger (see table 1); zygomatic breadth, breadth of braincase, occipitonasal length, least interorbital constriction, and length of rostrum all averaging larger (see table 2). From _Baiomys musculus grisescens_ (paratypes, from Comayabuela, Honduras) to the south, _B. m. handleyi_ differs in: buff-colored hairs in dorsal and ventral region lacking; fore- and hind-feet white, not flesh-colored with gray overtones; tail bicolored, not unicolored; face paler, lacking buff-brown coloration; anterior tips of nasals squared rather than flaring; tail and upper molar tooth-row significantly longer (see Table 1); hind foot, ear from notch, and rostrum longer; braincase averaging broader (see Table 2). _Remarks._--The occurrence of these pale mice in the Río Negro Valley was first noted by Goodwin (1934:39, 40) when he referred specimens from Sacapulas and Chanquejelve, Guatemala, to _B. m. musculus_. Hooper (_op. cit._:92-94) correctly assigned specimens from the southern coast and eastern part of Chiapas to _B. m. nigrescens_. The continued assignment of specimens from Sacapulas, Guatemala, to the subspecies _musculus_ produces a hiatus both in the range of _B. m. nigrescens_ and _B. m. musculus_. Twenty-four specimens, 14 from 1 mi. S Rabinal, and 10 from 1/2 mi. N, 1 mi. E Salama, Guatemala, are intergrades between _handleyi_ and _grisescens_, but show more resemblance to the latter and, therefore, are referred to that subspecies. To the north, _handleyi_ intergrades with _nigrescens_. The specimen from Chanquejelve is an intergrade between the two subspecies just mentioned.