Measurements of the energy cost of echolocation in stationary pipistrelle bats P. pipistrellus, using standard respirometry, indicate that there is a very high energy cost to ultrasonic vocalisation in air. Yet measurements of the energy expenditure of echolocating bats in Right, using doubly-labelled water, suggest no increase in cost above that of non-echolocating bats and birds during Right. The reason for this discrepancy in the apparent energy costs of echolocation may reflect the utilisation of the flight muscles not only to power the wing flapping but also ventilation and thus indirectly the echolocation pulses. Several species of bats not only echolocate in flight but also from stationary positions. The costs of echolocation in two of these species (Rhinolophus ferrumequinum and R. hipposideros at rest are considerably lower than for the stationary pipistrelle. This apparent efficiency may reflect two different processes. First the bats batch pulses together thus obtaining several pulses from each breath. Second the bats may have evolved the ability to decouple the generation of the ventilation pulse from contraction of the pectoral muscles.