_Comparisons._--From _B. m. nigrescens_, _B. m. handleyi_ differs as follows: everywhere paler; forefeet and hind feet whitish instead of dusky to sooty; hairs of anterior part of face white instead of brown; tail bicolored instead of unicolored; anterior tips of nasals truncate rather than rounded; frontoparietal suture forming obtuse angle with suture separating parietals instead of forming right angle; tail and upper molar tooth-row longer. From _B. m. grisescens_, _B. m. handleyi_ differs in: slightly paler above and below, primarily as a result of lacking buff-colored hairs; forefeet and hind feet white, not flesh-colored with gray overtones; tail bicolored, not unicolored; anterior tips of nasals truncate rather than flaring; tail and upper molar tooth-row longer. _Remarks._--_B. m. handleyi_ seems to be restricted to the valley of the Rio Negro, in the region of Sacapulas, Guatemala. Stuart (1954:7) points out that the Rio Negro drops down into a gorge at a place near Sacapulas and flows northward through a deep canyon for approximately 60 kilometers. The Rio Negro, then, flows onto the lowlands of the Yucatan Peninsula. The habitat is xerophytic in the valley of the Rio Negro near Sacapulas. Stuart (_op. cit._:10) suggests that this xerophytic habitat may be continuous to a place to the north of Chixoy, Chiapas, where the vegetation then becomes more mesic. The mesic conditions to the north in Tabasco and Yucatan probably have restricted the movement of pygmy mice to the north. No specimens of this mouse are known from the Yucatan Peninsula or from the State of Tabasco, Mexico. _B. m. handleyi_ intergrades with _B. m. grisescens_ to the south. Specimens from 1 mi. S Rabinal, and those from a second locality 1/2 mi. N and 1 mi. E Salama, Guatemala, are intermediate in color of pelage between _handleyi_ and _grisescens_. Stuart (_op. cit._:5) mentions the continuity of habitat and tributaries from the Salama Basin into the valley of the Rio Negro. Absence of physiographic and biotic barriers in the corridor between these two basins probably allows for some gene flow between _handleyi_ and _grisescens_, and results in populations intermediate in color. To the north and northwest of Sacapulas, the Sierra de los Cuchumatanes rises abruptly and separates the known geographic range of _handleyi_ from that of _nigrescens_ to the north, while to the west the cactus-mesquite habitat of _handleyi_ gives way to the oak-pine timber that, so far as known, does not support _Baiomys_. The difference in elevation and flora seems to restrict gene flow between _handleyi_ and the more northern _nigrescens_. The only evidence of integration between these two subspecies is provided by one specimen from Chanquejelve, Guatemala. That specimen is intermediate in color between the pale _handleyi_ and blackish-brown _nigrescens_.