We investigated the behavioural changes of Indo-Pacific bottlenose dolphins Tursiops aduncus in response to boat-based tourism at both group and individual levels. The behaviour, movement and dive patterns of nursing females off the south coast of Zanzibar were investigated between January and March 2000 to 2002 and statistical comparisons were made between observations made at different levels of tourist activity. Behavioural data was collected during boat surveys using scan sampling of groups and focal individual follows of 5 female dolphins with calves. The movement patterns of dolphin groups were not affected by the presence of a few (1 to 2) tourist boats without swimmers. However, the groups displayed a significantly larger proportion of erratic (non-directional) movements as tourist activities increased and when swimmers were present. The proportion of active, peduncle, tail-out and porpoise dives also increased as tourist activity increased. Further, females travelled more frequently as tourist activities increased; this may have a negative effect on the time available for females to nurse their calves. Intense non-regulated dolphin tourism in this area may lead to a shift in habitat use by nursing females, and the apparent changes in dolphin behaviour due to the increased levels of tourism may ultimately reduce fitness at both individual and population levels. We urge that the guidelines already issued by the Department of Fisheries and Marine Products, Zanzibar, be implemented and complied with as a first important step towards sustainable dolphin tourism.