We investigated the bat defence reactions of three species of moths (Gynaephora groenlandica, Gynaephora rossi (Lymantriidae) and Psychophora sabini (Geometridae)) in the Canadian Arctic archipelago. Since these moths inhabit the Arctic tundra and, therefore, are most probably spatially isolated from bats, their hearing and associated defensive reactions are probably useless and would therefore be expected to disappear with ongoing adaptation to Arctic conditions. When exposed to bat–like ultrasound (26 kHz and 110 dB sound pressure level root mean square at 1m) flying male Gynaephora spp. always reacted defensively by rapidly reversing their flight course. They could hear the sound and reacted at least 15–25 m away. Psychophora sabini walking on a surface froze at distances of at least 5–7 m from the sound source. However, two out of three individuals of this species (all males) did not respond in any way to the sound while in flight. Hence, we found evidence of degeneration of bat defence reactions, i.e. adaptation to the bat–free environment, in P. sabini but not in Gynaephora spp. Some Arctic moths (Gynaephora spp.) still possess defensive reactions against bats, possibly because the selection pressure for the loss of the trait is such that it declines only very slowly (perhaps by genetic drift) and there may not have been enough time for the trait to disappear. One possible reason may be that Arctic moths have long generation times.