The commonest flea on the prairie vole is _Ctenophthalmus pseudagyrtes_; it averages 1.9 (for subadult voles) to 3.4 (for adult voles) per individual vole. Other fleas on this vole are _Orchopeas leucopus_, _Orchopeas howardii_, _Nosopsyllus fasciatus_, _Epitedia wenmanni_, and _Rectofrontia fraterna_. The two species of fleas which were actually common on the vole (_C. pseudagyrtes_ and _O. leucopus_), parasitized also some other small mammals which lived in the same habitat as the vole. One species of sucking louse (_Hoplopleura acanthopus_) and two kinds of mites (_Laelaps kochi_ and _Atricholaelaps glasgowi_) which occur on the prairie vole in Kansas, occur also on _Microtus californicus_ in California and on _M. pennsylvanicus_ in New York. Only three ticks (1 _Dermacenter variabilis_ and 2 _Ixodes sculptus_) were found on the prairie vole. Fifty-eight gravid females had an average of 3.4 embryos. Litters at the height of the breeding season are larger than those at the beginning and at the end of the breeding season. Reproduction in _Microtus ochrogaster_ ceased in December, 1945, in northeastern Kansas, and the first evidence of reproduction in 1946 was observed in February. LITERATURE CITED BAILEY, V. 1900. Revision of the American voles of the genus Microtus. N. Amer. Fauna, 17:1-88, June 6, 1900. 1920. Identity of the bean mouse of Lewis and Clark. Jour. Mamm., 1:70-72, November 28, 1919. BAKER, J. R., and RANSOM, R. M. 1933. Factors affecting the breeding of the field mouse (_Microtus agrestris_). Part 11. Temperature and food. Royal Soc. London Proc., (Ser. B) 112:39-46, November 1, 1932. BOLE, B. P., JR., and MOULTHROP, P. N. 1942. The Ohio Recent mammal collection in the Cleveland Museum of Natural History. Sci. Pub. Cleveland Mus. Nat. Hist., 6:83-181, September 11, 1942. CRIDDLE, S. 1926. Habits of Microtus minor in Manitoba. Jour. Mamm., 7:193-200, August 9, 1926. ELTON, C. S., E. B. FORD, J. R. BAKER, and A. D. GARDNER. 1931. The health and parasites of a wild mouse population. Proc. Zoöl. Soc. London, 101:657-721, September 30, 1931. FISHER, H. J. 1945. Notes on voles in central Missouri. Jour. Mamm., 26:435-437, November, 1945. HATFIELD, D. M. 1935. A natural history study of Microtus californicus. Jour. Mamm., 16:261-271, November 15, 1935. HAMILTON, W. J., JR. 1937a. The biology of microtine cycles. Jour. Agr. Res., 54:779-790, May 15, 1937. 1937b. Growth and life span of the field mouse. American Nat., 71:500, September-October, 1937. 1941. The reproduction of the field mouse, Microtus pennsylvanicus (Ord). Cornell Univ. Agr. Exp. Sta. Memoir 237, pp. 1-23, May, 1941. KENNICOTT, R.