Table 5 shows the average degree of infestation for ten months of an eleven month period. The monthly averages for the most part show no variations. The latter half of February provides an exception in that a series of 22 snap-trapped voles and 11 live-trapped voles taken at that time had on the average, 9.7 and 5.3 fleas respectively. Pine mice (_Pitymys nemoralis_) occurred in small numbers in the area where _Microtus ochrogaster_ was live-trapped, and _Ctenophthalmus pseudagyrtes_ was the flea found to be common on both of these voles. TABLE 5.--_Monthly Averages of Fleas on Prairie Voles_ Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. .6 5.1 5[D] ... 3 1.8 1.4 1.7 ... 1.1 2 2 (6) (11) (6) ... (6) (88) (26) (6) ... (8) (14) (2) [Footnote D: This figure is high because one vole had the high number of 19 fleas. The numbers in parentheses show the number of live voles examined for each month. All fleas were _Ctenophthalmus pseudagyrtes_ Baker.] Some fleas have a habitat preference as well as a host specificity. As voles from different areas were examined, different kinds of fleas were encountered. A population of free-living voles under observation on the Campus at Lawrence was parasitized only by _Ctenophthalmus pseudagyrtes_. From 90 prairie voles collected in a field of clover 4 miles northwest of Lawrence, the only species of flea recovered was _Orchopeas leucopus_. In both places the prairie vole was the most common mammal, but in the field of clover three deer mice (_P. maniculatus_) also were trapped. In a third field, one mile west of Lawrence, the prairie vole was host to both the above mentioned fleas. Here both the prairie vole and the cotton rat (_Sigmodon hispidus_) were common. The host distribution of fleas on seven small mammals which lived in the same habitats as the prairie vole is given in table 6. TABLE 6.--_Frequency of Occurrence of Fleas on Seven Species of Small Mammals_[E] Column headings: A: _Cryptotis parva_ B: _Blarina brevicauda_ C: _Peromyscus maniculatus_ D: _Peromyscus leucopus_ E: _Sigmodon hispidus_ F: _Microtus ochrogaster_ G: _Pitymys nemoralis_