Frequency-modulated sweep calls of Otomops martiensseni were recorded from individuals as they emerged from nine different building roosts near Durban, South Africa. Multiple analyses of call features, including duration (ms), lowest frequency (kHz), highest frequency (kHz) and frequency with most energy (kHz), indicated significant inter-individual variation. Discriminant function analysis of call features correctly classified the calls of individuals from four roosts > 70% on 19 of 28 times. Although other species of molossids (Chaerephon pumilus, Tadarida aegyptiaca, and one unidentified species) produced social calls as well as frequency-modulated sweep calls, O. martiensseni produced just the latter vocalizations and they were longer and lower in frequency than those of the sympatric molossids. Other species of molossids, but not O. martiensseni, produced feeding buzzes as they attacked flying insects. The frequency-modulated sweep calls of O. martiensseni seem to serve a communication function, but they may not be used in echolocation unlike similar calls by other molossids. Individually distinct communication signals (frequency-modulated sweep calls) enhance communication in a species that lives in year-round social groups (one adult male, females and dependent young).