Experimental spotlighting of deer (Odocoileus virginianus and 0. hemionus) in meadows along a 68-mile route in the Black Hills, South Dakota, during the summer of 1958 indicated that the technique can be a means of studying population trends. Herd composition, availability of preferred foods in meadows, type and size of meadow, hour of observation, and several meteorological factors influencd nightly counts. Time can be standardized if counts are made only during a 4-hour period beginning 1 hour after sunset; nearly 79 percent of the 25,808 deer appearances were observed within this period. Over 82 percent were noted within 5 hours after sunset. Correlation of the combined effects of temperature, cloud cover, precipitation, dew, and relative humidity with deer counts was high. Temperature exerted the strongest influence on deer behavior. Statistical analysis of data from a selected portion of the route revealed a curvilinear relationship between temperature, day of season, and deer counts. The derived equation can be used for predicting counts for a given date at a given temperature. The technique of spotlighting open areas to count deer where they congregate at night has limited use on most deer ranges; however, the method could be a useful means for detecting changes in deer-population densities on some ranges if the relationship of factors affecting deer behavior, and therefore affecting nightly counts, are understood. This report evaluates several such factors studied during summer spotlighting of meadows in the Black Hills of South Dakota in 1958. Biologists in South Dakota have found that spotlight counts of deer are indicative of area deer population (Berner, L. M. 1953. Deer management in the Black Hills of South Dakota. Annual Meeting, Soc. Am. Foresters, Colorado Springs. Mimeo. 23pp). Previous to this study, spotlighting was tried in spring and fall, but counts of deer were erratic, primarily because of seasonal movements of local herds. Other states have attempted to census deer by spotlighting, but their programs have generally failed to yield reliable data (Personal communication from R. J. Boyd 1958, R. S. Cook 1958, R. L. Dawning 1958, L. L. Eberhardt 1958, J. L. Egan 1958). The 68-mile spotlighting route, divided into seven convenient segments (Fig. 1), transected various kinds of terrain, vegetative cover, and land-use patterns. It extended upward in elevation from 5,000 feet near Hill City to 6,800 feet northwest of Deerfield. Over 56 miles of the route ran through meadows which averaged about 200 yards in width on either side of the road. One portion of the route ran through 30 miles of continuous meadow. Most of the bottomlands along the route are privately owned and are cultivated for oats (Avena sativa), alfalfa (Medicago sativa), and other hays (Fig. 2). Sheep and cattle are pastured on some of them as well as on public lands along the route. The latter are predominantly upland prairies and forests (Fig. 3). Uncultivated meadows generally support dense growths of mixed grasses, mainly bluegrass (Poa pratensis), western wheatgrass (Agropyron Smithii), and crested wheatgrass (Agropyron sp.), along the creeks, especially in beaver flows. The ecotone between the dense ponderosa pine (Pinus ponderosa )-Black Hill spruce (Picea glauca) forest and the meadows is dominated by aspen (Populus tremuloides) and paper birch (Betula papyrifera). The