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ABSTRACT: Vessel disturbance is one potential risk factor to the endangered population of southern resident killer whales Orcinus orca. This study was conducted to determine if southern resident killer whales perform surface active behaviors (SABs) in response to close approaches by vessels. Data were collected in the San Juan Islands, USA, and Gulf Islands, Canada, from May through September 2005 and 2006. Continuous behavioral data, including the performance of SABs (e.g. spy hops, breaches, tail slaps, pectoral fin slaps), were recorded from southern resident killer whales using a focal follow approach. Distances between the focal whale and nearby vessels were systematically measured throughout each focal follow. In addition, the distance between the nearest vessel and the focal whale was recorded each time the whale performed an SAB. Tail slaps were the most frequently performed SAB. The highest frequency of SABs occurred when the nearest vessel was within 75 to 99 m and 125 to 149 m of the focal whale in 2005 and 2006, respectively. Approximately 70% of SABs occurred when the closest vessel was within 224 m of the whale. Furthermore, a significantly greater proportion of SABs occurred when vessels closely approached whales. Finally, there was a significant temporal relationship between close approaches and the occurrence of SABs; most SABs were performed near the time of the closest approach by a vessel. These results suggest that close approaches by vessels elicit behavioral responses in southern resident killer whales and that the minimum approach distance of 100 m in whale-watching guidelines may be insufficient in preventing behavioral responses from whales.