Responses of the ragwort Ilea beetle,Longitarsus jacobaeae(Waterhouse), to cues associated with conspecific beetles were examined using two-choice tests in the laboratory and in the field. In the laboratory test, a male or female flea beetle was allowed to choose between two tansy ragwort (Senecio jacobaeaL.) leaves, one that had been previously exposed to five male or female beetles for 1 d, and the other without any previous exposure to conspecific beetles. Male beetles were found more often on leaves that had been previously exposed to female beetles than on the clean leaves without female-associated cues, but were randomly distributed when the treated leaves were previously exposed to male beetles. Female beetles were randomly distributed between the two leaves, regardless of whether one leaf was previously exposed to male beetles or female beetles. Similar tests using filter paper as substrate instead of leaves showed similar results, suggesting that male beetles responded to cues associated with female beetles rather than to plants. In a field experiment, ragwort leaves that had been previously exposed to female beetles attracted more male beetles than leaves without female-associated cues. The evidence suggests thatL. jacobaeaefemales emit a sex pheromone that is attractive to male beetles. Attraction of male beetles to leaves exposed to females may be one of the mechanisms underlying the clumped distribution of flea beetles within plants.