ECHOLOCATION has evolved in relatively few animal species1. One constraint may be the high cost of producing pulses, the echoes of which can be detected over useful distances2. The energy cost of echolocation in a small (6 g) insectivorous bat, when hanging at rest, was recently measured at 0.067 Joules per pulse3, implying a mean cost for echolocation in flight of 9.5 × basal metabolic rate (range 7 to 12×). Because flight is very costly4, whether the costs of echolocation and flying are additive is an important question. We measured the energy costs of flight in two species of small echolocating Microchiroptera using a novel combination of respirometry and doubly-labelled water5. Flight energy expenditure (adjusted for body mass) was not significantly different between echolocating bats and non-echolocating bats and birds. The low cost of echolocation for flying vertebrates may have been a significant factor favouring its evolution in these groups.