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During the austral summer of 2002, theodolite tracking was used to evaluate Chilean dolphin (Cephalorhynchus eutropia) behavioural responses to boats in Yaldad Bay, southern Chile. This bay represents an important site for the occurrence of this species. Boat traffic has increased considerably since 1980 in this area due to aquaculture activities. Behavioural responses were analysed for each dolphin activity, and pre-, during, and post-boat encounters. When foraging and approached by a vessel, dolphins increased their reorientation rate, whereas swimming speed showed no significant change. When traveling, however, dolphins reacted to boats by increasing their directional swimming speed, while reorientation rate did not differ. After encounters, dolphins seemed to return quickly to previous behavioural patterns when traveling, whereas it took longer to establish normal patterns when foraging. Group dispersion analyses showed that when boats approached foraging dolphins, they became significantly more cohesively grouped. Consequently, dolphins reacted negatively to boat presence in Yaldad Bay, but these responses were conditional on dolphin behavioural activities prior to boat encounters. These findings emphasize the need to consider boat traffic disturbance on cetaceans in coastal management plans.