Like other small terrestrial mammals, bats have a high mass-specific energetic demand because of the fact that they have an unfavorable surface area to volume ratio. Furthermore, bats have a very energy-expensive mode of locomotion: flight. This high energetic demand has to be covered by food intake. The retention time of the digestive tract is one factor affecting the energy intake of bat species. Factors like energy demand, gut volume and dietary specialization influence retention time in mammals. However, maximum retention time for only Myotis myotis and transit time only for M. lucifugus, Nyctophilius gloudi and Nyctalus noctula is known. This study investigated the maximum retention times and transit times of 10 Central European bat species. It was hypothesized that the level of specialization of the digestive tract, energy-demanding processes and intestine length would affect the retention time of bats. Fluorescence-marked mealworms Tenebrio molitor were used to measure the time between the first ingested mealworm and the first appearance of the marker or the last fluorescing feces, respectively. For the first time, the retention time of 10 insectivorous bat species was measured to determine interspecific differences. Additionally, we measured the retention time of post-lactating female and spermatogenically active male Pipistrellus pipistrellus to determine intraspecific differences. The retention time of bats differed significantly between species and is probably influenced by the level of specialization of the digestive tract. High-level specialization of the digestive tract resulted in short retention times. Furthermore, significant intraspecific differences between post-lactating and spermatogenically active individuals of P. pipistrellus showed that the retention time within a single species might be influenced by energy-demanding processes (e.g. reproduction).