We evaluated composition and seasonal variation in the diet of Rickett's big-footed myotis (Myotis ricketti) by examining 342 fecal samples collected every 2 weeks when the bats were active in Fangshan District of Beijing, North China, from 2002 to 2003. The diet consisted of 3 kinds of fish (Zacco platypus, Carassius auratus, and Phoxinus lagowskii) and at least 7 orders of insects (Coleoptera, Lepidoptera, Homoptera, Ephemeroptera, Hemiptera, Diptera, and Hymenoptera). Fragments of fish accounted for 67.1% and fragments of insects for 28% (by volume) of the diet; Z. platypus was the dominant food (60.1%), followed by Coleoptera (13.1%). Significant differences existed between the proportion of fish and insects in the diet; however, no seasonal variation in the diet was found over the course of the investigation. This suggests that at this study site in Beijing, M. ricketti was highly specialized in foraging on only 1 species of fish and relied less on insects as alternative food sources, despite their seasonally high abundance.