In young females sterile oestral cycles often begin at about the time of weaning. Earliest pregnancies occur when females are approximately one month old, but most are several weeks older before they become pregnant. Rate of growth declines steadily from a length increment of approximately 2 mm. per day in voles less than two weeks old to an increment of approximately one-fourth mm. per day in subadults. Growth rate is highly variable among individuals at all stages, and especially in those that have attained adult size. Even adults tend to gain in length, slowly, as well as in weight, and the largest individuals are all many months old. LITERATURE CITED BAILEY, V. 1924. Breeding, feeding and other life habits of meadow mice. Jour. Agric. Res., 27: 523-536. BODENHEIMER, F. S., and F. SULMAN. 1946. The estrous cycle of _Microtus guentheri_ D. and A. and its ecological implications. Ecol., 27: 255-256. GREENWALD, G. S. 1956. The reproductive cycle of the field mouse, _Microtus californicus_. Jour. Mamm., 37: 213-222, 2 figs., 1 pl. _Hamilton, W. J., Jr._ 1941. The reproduction of the field mouse, _Microtus pennsylvanicus_. Cornell Univ. Agric. Exp. Sta. Mem., 237: 1-23. HATFIELD, D. M. 1935. A natural history of _Microtus californicus_. Jour. Mamm., 16: 261-271. HOYTE, H. M. D. 1955. Observations on some small mammals of Arctic Norway. Jour. Animal Ecology, 24: 412-425. JAMESON, E. W. 1947. Natural history of the prairie vole. Univ. Kansas Mus. Nat. Hist. Publ., 1: 125-151. MARTIN, E. P. 1956. A population study of the prairie vole (_Microtus ochrogaster_) in northeastern Kansas. Univ. Kansas Mus. Nat. Hist. Publ., 8: 361-416. SCHMIDT, F. J. W. 1931. Mammals of western Clark County, Wisconsin. Jour. Mamm., 12: 99-117. [] 26-7561 UNIVERSITY OF KANSAS PUBLICATIONS MUSEUM OF NATURAL HISTORY Institutional libraries interested in publications exchange may obtain this series by addressing the Exchange Librarian, University of Kansas Library, Lawrence, Kansas. Copies for individuals, persons working in a particular field of study, may be obtained by addressing instead the Museum of Natural History, University of Kansas, Lawrence, Kansas. There is no provision for sale of this series by the University Library which meets institutional requests, or by the Museum of Natural History which meets the requests of individuals. However, when individuals request copies from the Museum, 25 cents should be included, for each separate number that is 100 pages or more in length, for the purpose of defraying the costs of wrapping and mailing. * An asterisk designates those numbers of which the Museum's supply (not the Library's supply) is exhausted. Numbers published to date, in this series, are as follows: Vol. 1. Nos. 1-26 and index. Pp. 1-638, 1946-1950.