Audiograms and behavioural responses to ultrasound reveal that male geometrid winter moths (Agriopis and Erannis spp.; Ennominae, and Alsophila aescularia; Oenochrominae), which have large wings and a slow flight, have good, broadly tuned ultrasonic hearing with best frequencies at 25-40 kHz, coinciding with the frequencies used by most sympatric aerial-hawking bats. Ultrasonic pulses (27 kHz 110 dB at 1 m) delivered at distances of 1-12 m evoked consistent reactions of free flying, male A. marginaria in the lab as well as in the field; those at < 5 m resulted in the moth spiralling or diving towards the ground, those at 5-12 m resulted in one or several changes in the flight path, but did not end on the ground. The differential reaction probably reflects whether the moth is likely to have been detected by the bat or not. The micropterous (and flightless), and hence cryptic, females have strongly reduced tympanic organs and are virtually deaf. Sexual dimorphism in hearing and behavioural reactions to ultrasound reflect differential natural selection on males and females by bats. Natural selection on the hearing of the males thus seems to occur although they fly in late autumn and early spring, when bat activity is much reduced.