Seasonal variations in the diet composition and prey selection by Myotis myotis were studied in a Mediterranean region dominated by open stone oak woodlands, olive groves, and cereal steppes. The diet and food abundance were determined by faecal analysis and pitfall trapping, respectively. Overall, the diet (expressed as % frequency) was dominated by carabid beetles (Carabidae, 52%), crickets (Gryllidae, 43%), and spiders (Arachnida, 34%). Food was far more abundant in the spring than in the summer and autumn. The dietary composition varied significantly throughout the year. In the spring, it was dominated by carabids and crickets, and in the summer by spiders. These were then replaced by carabids, which became the most frequently eaten prey in autumn. The spring-summer switch to spiders coincided with a marked drop in food abundance. Some of the temporal variations in bats' diet can be explained by parallel changes in prey abundance. In comparison to central European populations, our Mediterranean colony of M. myotis consumed much more crickets and spiders, which was balanced with a lower use of carabid beetles. Prey selection was tested using logistic regression models. The results suggest that, although carabids are the most consumed, crickets are preferred. Spiders seem to represent mostly a complement preyed upon at times of lower food abundance. It is not clear if the preference for crickets is simply due to their great conspicuousness, or to the choice based on energetic rewards or taste. Overall, our results are consistent with the predictions of the optimal foraging theory.