During the summer of 1979 a cave which served as a summer aggregation site for male Indiana bats (Myotis sodalis) was visited every 2 weeks for collection of feces and sampling of the insect fauna available as prey. A comparison of insects eaten, determined by fecal analysis, with insects collected from the foraging habitat showed that large proportions of lepidopterans were eaten throughout the summer, often in excess of proportional availability. Insects from five additional orders were eaten regularly, but they were frequently eaten less than the proportion in which they were available. Throughout the season, dietary intake of aquatic insects was low. Diet diversity decreased from early to late summer. The proportions of several orders of aquatic insects eaten were correlated to three measures of lunar illumination and cyclicity.