Studies on captive bottlenose dolphins, Tursiops truncatus, have shown that each individual produces a stereotyped, individually specific signature whistle; however, no study has demonstrated clear context-dependent usage of these whistles. Thus, the hypothesis that signature whistles are used to maintain group cohesion remains untested. To investigate whether signature whistles are used only in contexts that would require a mechanism to maintain group cohesion, we examined whistle type usage in a group of four captive bottlenose dolphins in two contexts. Individuals were recorded while they were separate from the group and while they all swam in the same pool. Separations occurred spontaneously when one animal swam into another pool. No partitions were used and no aggressive interactions between dolphins preceded separations. Calling animals were identified by an amplitude comparison of the same sound recorded in the two pools. Each dolphin primarily produced one stereotyped signature whistle when it was separated from the group. Similarly the remaining group in the other pool also used primarily their signature whistles if one animal was in a separate pool. If all animals swam in the same pool almost only nonsignature whistles were used. Signature whistle copying was rare and did not initiate reunions or specific vocal responses. The results strongly support the hypothesis that signature whistles are used to maintain group cohesion. Copyright 1998 The Association for the Study of Animal Behaviour.