The diets of seven of the eight species of insectivorous, vespertilionid bats found in Tasmania were studied in forest areas. The diet of the eighth species, Chalinolobus gouldii, was studied in riparian woodland surrounded by open pasture. Nyctophilus timoriensis was found to be a gleaner with most of its diet consisting of non-volant insects. Five of the species (Eptesicus vulturnus, Eptesicus regulus, Eptesicus sagittula, Nyctophilus geoffroyi and Chalinolobus morio) fed opportunistically, their diets reflecting local and temporal variations in insect abundance and diversity. Lepidoptera made up most of the diet of these species. The largest species, Falsistrellus tasmaniensis, appeared to be a selective feeder, with Coleoptera being the most important item in its diet. Coleoptera also made up the bulk of the diet of C. gouldii, but caterpillars were also important. For the species studied in forest areas, there was a highly significant correlation between the mean sizes of the lepidopterans (r= 0.95) and coleopterans (r = 0.96) eaten and the size of the bat. For all bats larger than E. sagittula, the mean size of beetles eaten was smaller than the mean size of moths eaten. These results are in accord with expectations from optimal foraging theory.