In the course of live-trapping operations several species of small mammals other than _Microtus ochrogaster_ were taken in the traps. Also, from time to time, direct observations of certain mammals were made and various types of sign of larger mammals were noted. These records gave a picture of the mammalian community of which the voles were a part. The three associated species which were most commonly trapped were _Sigmodon hispidus_, _Reithrodontomys megalotis_ and _Peromyscus leucopus_. These three species have been commonly found associated with _Microtus_ in this part of the country (Fisher, 1945:435; Jameson, 1947:137). The Texas cotton rat, _Sigmodon hispidus_, was the most commonly trapped associate of the voles between November, 1950, and February, 1952. Although a greater number of individuals of the harvest mouse were taken in a few months, the cotton rat had a greater ecological importance because of its larger size (Figs. 17, 18, 19). The cotton rat was an especially noteworthy member of the community for two reasons. It has arrived in northern Kansas only recently and its progressive range extension northward and westward has attracted the attention of many mammalogists (Bailey, 1902:107; Cockrum, 1948; 1952:183-187; Rinker, 1942b). Secondly, _Sigmodon_ has long been considered to be almost the ecological equivalent of _Microtus_ and to replace the vole in the southern United States (Calhoun, 1945:251; Svihla, 1929:353). Since the two species are now found together over large parts of Kansas their relationships in the state need careful study. [Illustration: FIG. 17. Variations in density and mass of three common rodents on House Field. The upper graph shows the sum of the biomass of the three rodents. In the two lower graphs the solid line represents _Microtus_, the broken line _Sigmodon_, and the dotted line _Reithrodontomys_.] [Illustration: FIG. 18. Variations in density and biomass of three common rodents on Quarry Field. For key, see legend of Fig. 17.] [Illustration: FIG. 19. Changing biomass ratios of three common rodents on House Field and Quarry Field. In late 1951 and early 1952 the cotton rats attained relatively high levels and seemingly caused compensatory decreases in the numbers of voles. The solid line represents _Microtus_, the broken line _Sigmodon_, and the dotted line _Reithrodontomys_.]