The population of _Sigmodon_ retained its high level, relative to _Microtus_, until February, 1952. In March only one individual was captured and after that none was trapped until August, 1952, when a single subadult male was captured. Early in March, 1952, before the trapping period for the month had begun, the area suffered three successive days of unusually low temperature, with snow, which lay more than six inches deep in places. As suggested by Cockrum (1952:185), such conditions proved detrimental to the cotton rats and, at least to the end of the study period in August, 1952, the population of cotton rats had failed to recover. Perhaps the extremely dry weather which followed the heavy winter mortality delayed the recovery of the population. These limited data seem to indicate competition between _Sigmodon_ and _Microtus_ in Kansas. Extremely wet conditions seem to give _Sigmodon_ a competitive advantage whereas _Microtus_ is better able to survive dry summers and severe winters. However, these relationships need further clarification by an intensive study of the life history of _Sigmodon_ in Kansas (especially the more arid western part), including its coactions with the communities it has invaded successfully recently. The harvest mouse (_Reithrodontomys megalotis_) also was a common inhabitant of the study plots, but this small rodent seemed not to be a serious competitor of the voles, as its food consists almost entirely of seeds (Cockrum, _op. cit._:165) not usually used by voles. In this study, at least, no conflict over space was apparent. Harvest mice frequently were taken in the runways of voles and even in the same trap with voles. Reithro Field, the part of the Reservation having the heaviest population of the harvest mouse, differed from the habitats that were better for voles in being higher, drier and less densely covered with vegetation. However, during the summer of 1951 when the voles were most abundant, Reithro Field supported a large population of voles. Estimates of population of the harvest mouse were of doubtful validity in summer because it was readily trapped only in winter and early spring. Many individuals marked in late spring were not trapped again until late autumn although presumably they remained on the area. This seasonal variation in trapping success seemed to be a matter of acceptance and refusal of bait (Fitch, 1954:45). The presence of the wood mouse (_Peromyscus leucopus_) on the study plots indicated an overlapping of habitats. Both House and Quarry Fields were on the ecotone between forest and meadow and a mixture of mammals from both types of habitat occurred. No sign of the homes of the wood mouse was found on the study plots, and on the larger trap line, operated by Fitch, wood mice were captured only near the edge of the woods.