The function of social calls emitted by foraging bats has received little study. Here we use observations of free-ranging greater spear-nosed bats, Phyllostomus hastatus, and field playbacks to determine whether audible, broad-band 'screech' calls attract mates, warn conspecifics or influence access to food. Five lines of evidence suggest that screech calls enable adult females from the same roosting group to fly together from the day roost to feeding sites. (1) Seasonal differences in diet influenced the rate of screech calling recorded outside the cave roost, as well as how often bats departed together. Bats called more often and flew in larger groups when feeding on a concentrated resource, balsa, Ochroma lagopus, flowers, in winter than on more dispersed Cecropia peltata fruit in spring. (2) Observations of bats flying outside the cave, in flyways and at feeding sites indicated that screech calls occurred more often when bats flew in groups than alone. (3) Females from the same roosting group were netted at the same feeding site, sometimes simultaneously, several kilometres from the cave. (4) Calling colour-marked adult females outside the cave were joined by a female group member, both on initial departures and on second foraging trips, more often than non-calling bats. (5) Playbacks attracted conspecifics at roost and feeding sites. Screech calls appear to function as contact calls that recruit and coordinate foraging among group members. We postulate that females benefit from foraging with unrelated roost-mates because they can defend feeding sites more effectively. Copyright 1998 The Association for the Study of Animal Behaviour.