Long-call vocalizations have been hypothesized to serve as communicative signals both within and between tamarin species. Prior evidence for the use of long calls as interspecific signals, however, is based solely on observations of apparent countercalling between species. I conducted playback experiments with groups of wild saddle-back tamarins, Saguinus fuscicollis, and emperor tamarins, S. imperator, to test the hypothesis that long calls function as interspecific signals between tamarin species forming polyspecific associations. Results show that in the absence of any other external cues, lone groups of saddle-back tamarins and emperor tamarins respond to the playback of long calls from both conspecific group members and heterospecific tamarins with which they associate. Playback of long calls from heterospecific tamarins elicited long calls and approaches towards the speaker. These findings are consistent with the hypothesis that long calls serve as interspecific signals between associating tamarin species.