_Diagnostic characters._--A short-tailed _Reithrodontomys fulvescens_ having a distinctly streaked or "peppered," and short pelage composed of relatively dark hairs as well as relatively brightly colored hairs on dorsum and white-tipped hairs on venter; and having shallow skull, elongate and posteriorly attenuate incisive foramina, small postpalatal foramina, broad interorbital region, and mid-dorsal depression at junction of nasal and frontal bones. _Comparisons._--Each of three adults of _meridionalis_ (trapped in February and March) can be distinguished from seven adults of the geographically adjacent _R. f. chiapensis_ from Guatemala (trapped in late January and in March) by shorter tail, more streaked or "peppered" dorsal pelage, yellower hue of non-blackish parts of hairs on dorsum, less distinct mid-dorsal darkening, and more whitish (less buffy) venter. Four specimens of _meridionalis_ trapped in July at the type locality, only one mile from the locality of capture of the February-taken specimens, are distinctly darker dorsally and slightly darker ventrally than the three _meridionalis_ trapped in February and March, but resemble the latter three in shortness of tail and in having short, "peppered" dorsal pelage. We lack specimens of _chiapensis_ in summer pelage. According to Hooper (1952:122) the summer pelages of _R. f. chiapensis_ and _R. f. helvolus_ are indistinguishable. Our _meridionalis_ differ from five summer-taken specimens of _helvolus_ in shorter, more "peppered," and distinctly darker dorsal pelage.