The pipistrelle (Pipistrellus pipistrellus) occurs as two phonic types in the British Isles, termed the 45 kHz phonic type and the 55 kHz phonic type. The two types are probably sibling species and therefore may have different diets. The aim of this study was to investigate differences in the diets of the two phonic types by using faecal analysis. Droppings were collected from three roosts of the 45 kHz phonic type and three roosts of the 55 kHz phonic type in three standardized time-periods during June and July 1994. Prey remains were identified in each sample as far as possible to the class, order, or family level, and the Percentage Items of each prey group in the diet of each phonic type was calculated. Analyses of variance were carried out to investigate the effects of time-period and roost on dietary composition, and to determine differences in the diets of the two phonic types for the main prey groups found. Both phonic types ate mostly the dipteran suborder Nematocera, and there was no difference between the phonic types in dietary breadth. The main prey groups in the diet of bats of the 45 kHz phonic type were the families Psychodidae, Anisopodidae and Muscidae, whereas the main prey groups of bats of the 55 kHz phonic type were the families Chironomidae and Ceratopogonidae. Differences in the diets of the two phonic types are discussed with respect to differences in habitat use by, and differences in echolocation call frequencies of, the two phonic types. The reported dietary differences support the hypothesis that the two phonic types are sibling species.