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ABSTRACT: The short-term effects of tourist boats on the behaviour of Indo-Pacific bottlenose dolphins Tursiops aduncus were investigated off the south coast of Zanzibar, Tanzania, by comparing dolphin group behaviour in the presence (impact) and absence (control) of tourist boats. Groupfollows were conducted from a carefully maneuvered (non-invasive) independent research vessel and behavioural data on group activity were collected using scan sampling methods. By using a timediscrete Markov chain model, the transition probabilities of passing/changing from one behavioural state to another were calculated and compared between impact and control situations. The data were further used to construct behavioural budgets. In the presence of tourist boats, dolphins were less likely to stay in a resting or socialising activity but were more likely to start travelling or foraging, as inferred from the Markov chain model. The behavioral budgets showed that foraging, resting and socialising all decreased as an effect of tourist boat presence, while travelling increased. The behavioural responses are likely to have energetic implications, mainly by increasing physical demands. Further, the results demonstrate that the current level of tourism intensity off the south coast of Zanzibar affects the dolphins’ cumulative behavioural budget. Regulations on dolphin tourism are therefore urgently needed to minimise potential long-term negative effects on the dolphins.