In the eighteenth century several natural historians suggested that the seemingly chaotic flight of moths was somehow related to attacks by hunting bats (see Kirby and Spence 1826), but the details of this relationship were clarified only after we under stood something about moth hearing and about bat echolocation, or bio sonar?the bat's use of echoes of the sounds it produces to detect objects from which these echoes rebound. Moths in several families have ears sensitive to ultrasonic sound located variously on their thoraxes, abdomens, or mouth parts (See Michelsen 1979), and these moths respond neurologically and behaviorally to the orientation cries of insectivorous bats. Moths that can hear such cries are 40% less likely to be caught than those that cannot (Roeder 1967), and there are similar data for some lacewings (Miller and Olesen 1979). The evasive response of moths is thus an example of a predator-specific defense (Edmunds 1974). Two main lines of study have contributed to our understanding of the interaction between moths and bats: a detailed consideration of what moths hear and a close examination of the echolocation, prey selection, and feeding behavior of bats (see Fig. 1). This research suggests that moths may, by their ability to detect mar auding bats, exert a selective pressure on echolocation strategies. Here we review the data on moth hearing and bat echolocation, analyze the implications of these data for detectability of bats by moths, and examine avail able information on the diets of insectivorous bats to find out if our predictions are correct. We also consider some of the intriguing questions raised by this research. If all bats are not equally detectable by all moths, do bats that are acoustically inconspicuous take advantage of this characteristic by specializing in moths? What is the function of the clicks produced by some tiger moths (Arctiidae) immediately before they are attacked by a bat?