It has long been recognized that microtines are essentially vegetarian in their food habits, but little has been learned about the exact plant species which comprise their diet because the finely chewed material has defied easy identification. Modification of existing techniques, based on the examination of the stomach contents of field-trapped animals, by the addition of a simple staining procedure makes it possible to identify much of this plant material to species.Increasing interest in the study of energy flux in ecosystems and continued effort to solve the riddle of microtine population fluctuations have emphasized the need for more specific information on microtine food habits than currently exists. The literature contains many references to the foods these rodents eat, but with few exceptions the statements are rather uninformative even when studies have been based on the examination of stomach contents (Hatt, 1930;Calhoun, 1941;Hamilton, 1941; Summerhayes, 1941;Quay, 1951).Perhaps the most useful study of microtine food habits has been the work of G. K. Godfrey (1953) on Microtus agrestis in Wytham Woods, Berkshire, England. She was able to distinguish epidermal fragments of a number of local grasses and a sedge in the fecal pellets from live-trapped voles. Recently, others (Dowding, 1955;Bakerspigel, 1956) have identified the mycelial threads and spores of the hypogeous fungus, Endogone, from the digestive tracts of some voles.The present study was undertaken to devise a technique which could be used to provide information on microtine food habits at least as specific as that available for Peromuscus, Mus, Zapus and other genera which do not chew their food as finely as do the voles (Calhoun, op. cit.; Hamilton, Ope cit.; Williams, 1965 Williams, , 1959a. The procedure developed is based on the method of determining dietary habits by the identification of fragments of plant epidermis found in the stomachs of field-trapped animals by comparison of the cell patterns of these fragments with those of known plant species. The incorporation of a simple staining routine makes possible the identification of this plant material with a degree of certainty previously unattainable in studies of this sort.Work on microtine food habits, started in 1957, has been supported in part by a grant from the Council on Research and Creative Work, University of Colorado. The Institute of Arctic and Alpine Research generously made its facilities available for much of the field work, and valuable assistance was rendered by Mr. Mark Paddock who collected most of the alpine plants from which the first set of reference slides of plant epidermal patterns was made. PROCEDURE The method by which the permanent slides of the epidermal layers of stems and leaves were prepared was developed from the techniques used by others employing plant histological methods in food habit studies (Baumgartner and
