Some authors (Verts, 1960:6, for instance) have recorded females of _R. megalotis_ as larger than males in external measurements, whereas others (Dalquest, 1948:325, for instance) have recorded males as the larger. In order to learn something of secondary sexual variation, and to decide whether or not to separate the sexes in our study, we compared adult males and females from the southern part of the Panhandle of Nebraska (Cheyenne, Keith, Kimball, Morrill and Scotts Bluff counties) in four external and twelve cranial measurements (see Table 1). The external measurements are those customarily taken by collectors and were read from the labels of the specimens; cranial measurements were taken to the nearest tenth of a millimeter by means of dial calipers, and are those described by Hooper (1952:9-11). Females from our sample averaged larger than males in all external and several cranial measurements, but individual variation greatly exceeded secondary sexual variation in each of these measurements and in no case was the greater size of females statistically significant. Therefore, and because we found no qualitative external or cranial differences between the sexes, males and females have been considered together in each population studied. TABLE 1. ANALYSIS OF SECONDARY SEXUAL VARIATION IN ADULT REITHRODONTOMYS MEGALOTIS FROM THE SOUTHERN PART OF THE NEBRASKA PANHANDLE. FOR EACH MEASUREMENT, THE NUMBER OF SPECIMENS USED, THE AVERAGE, THE EXTREMES, AND ONE STANDARD DEVIATION ARE GIVEN.