Until the late 1990s all Pipistrelle bats in the UK were thought to be one species. However, suspicions were first roused when it became clear that some Pipistrelles (now known as Common Pipistrelle) had a peak call around 45kHz, whilst others (Soprano Pipistrelle) had a peak call around 55kHz. Studies of the DNA confirmed they were two different species which can often (but not always) be identified by their different appearance: Common Pipistrelle tends to have a blackish ‘mask’ across the face. The Common Pipistrelle is virtually ubiquitous roosting in the centres of our towns as well as in the countryside. Maternity roosts are usually moderate in size (30-100 individuals) and located in a wide range of situations, including modern houses. Whilst individual bats or small numbers are sometimes found in winter (usually during building works) no large concentrations are encountered and the species is rarely found underground.