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We describe the ontogeny of pipistrelle bats Pipistrellus pipistrellus Schreber (Chiroptera: Vespertilionidae), including for the first time the development of true napping flight. The study animals were born to a group of 20 adults taken into captivity just before parturition, and allowed free flight and association within a room designed to approximate external conditions. All juveniles were aged to within one day and individually marked. All adults were ringed. Comparison to wild studies and the application of a set of growth models to forearm and body mass data gave no indication that development had been altered by captivity. Forearm data were best fitted by the logistic growth model and mass data by the ‘Gompertz’ growth model. Preliminary flight observations were followed, once the bats had become truly volant, by experiments in a flight enclosure with stroboscopic stereophotogrammetry. As the bats aged they used slower wingbeat frequencies (scaling with age D as D-0–40), but flew faster, speed scaling with age as D0–65. Wingbeat amplitude did not alter significantly with age, nor did the total mechanical power for flight, calculated by using a flight performance model, although the cost of transport fell as the bats grew older. This was probably due to the improving efficiency of the wing; the development of wingspan, wing area, wing loading, aspect ratio and tip area ratio are presented, and adaptations for reducing the energy requirements during early flights are discussed. These included a mass recession which occurred after the time of first flights. The flight model was also used to explore the hypothetical flight of bats with the morphology of neonates, and we discuss the extent of sexual dimorphism in the young bats and in their mothers.