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There is a great lack of information on the effects of boat operations on sperm whales (Physeter macrocephalus), which is the target species of the recent whale-watching industry in the Archipelago of the Azores. During the 1998 Azorean whale-watching season, between 4 June and 23 September, observations were carried-out from land-based lookouts and at sea from whale-watching boats to quantify short-term reactions of sperm whales to the presence and manoeuvres of boats. Results from landbased observations did not indicate changes in the behaviour of sperm whales, either due to the presence of boats or when exposed to inappropriate boat manoeuvres (as designated by proposed Azorean legislation). From boat-based observations, change in the whale’s speed and the presence of aerial displays were significantly more frequent when facing inappropriate boat manoeuvres. The presence of swimmers also led to a higher frequency of aerial displays by whales. In the presence of boats, mature females and immature individuals significantly increased their individual mean blow interval when accompanied by calves. Although some indications of disturbance were detected, we found no clear pattern of short-term reactions of sperm whales to whale-watching boats. It is strongly recommended that the activity is continuously monitored to assess its long-term effects, which generally remain unclear