The pipistrelle bat Pipistrellus pipistrellus is the most widespread in Europe, and is often the most abundant bat species in northern and central Europe. P. pipistrellus has traditionally been considered as one species throughout Europe. Here we show that: (i) the echolocation calls of pipistrelles fall into two distinct frequency bands in Britain, with frequencies containing most energy averaging 46 kHz and 55 kHz; (ii) roosts consist of bats which use only one form of echolocation; (iii) small but significant differences in average gross morphology exist between the phonic types; and (iv) the two phonic types occur in sympatry over much of Britain. Recordings made from European populations show that bimodal echolocation is also typical of continental pipistrelles, with only one phonic type found in some areas, two types in sympatry in others. The sympatric occurrence of two phonic types, the lack of mixing of types between colonies, and the morphological divergence between phonic types suggests that P. pipistrellus may actually consist of at least two cryptic sibling species. The evolution of bimodal echolocation in bats is discussed, and a model involving disruptive selection is presented to show how sympatric speciation may occur.