(Uploaded by Plazi for the Bat Literature Project) The study aimed to determine whether the so-called social calls that are produced by foraging bats of two phonic types of Pipistrellus pipistrellus serve a social function. First, the relationship between insect availability and the rate of production of social calls was measured at a foraging site of both phonic types. Second, playback experiments of social calls of the two phonic types were conducted in the field to determine the response of foraging bats to these calls, and to determine whether the calls are used in communication within or between phonic types. Two hypotheses are suggested for the function of social calls: that they may be used either to attract other bats to a food patch, or in agonistic interactions between bats in defence of a food patch. At relatively low insect densities, the rate of social call production of both phonic types increased significantly as insect density decreased. When social calls of each phonic type were broadcast, there was a significant reduction in bat activity of the same phonic type. In contrast, playbacks of social calls resulted in no change in activity of the other phonic type. The results supported the food patch defence hypothesis, that social calls are used to warn off other bats of the same phonic type when insects are scarce. The results also supported the hypothesis that the two phonic types are sibling species. Social calls were shown to serve a social function in intraspecific communication, but there was no communication between phonic types.