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The ecology and population biology of bottlenose and humpback dolphins inhabiting the Maputo Bay, Southern Mozambique were studied through boat based photo-identification surveys and behavioural methods between December 1995 and December 1997. Data from preliminary surveys carried out in 1992 are also included. Bottlenose dolphins occurred throughout the year in Maputo Bay, but were sighted infrequently (36 % of surveys). Their occurrence and group size were significantly larger during winter. Group size of bottlenose dolphins not differ significantly between months, daylight hours, semi-lunar tidal cycles and depth. Most identifiable bottlenose dolphin individuals had low site fidelity, but nursing females had relatively high site fidelity, implying that Maputo Bay is a nursing area. The group dynamics of bottlenose dolphins suggests a fluid (fission-fusion) social organisation. Age and sex appears to influence the degree of association between individuals,. An influx of bottlenose dolphins occurred during winter and influenced group size and occurrence. Bottlenose dolphins occurred in restricted areas of the Bay, preferring the north easter