Diving behaviour of air-breathing vertebrates may be influenced by a variety of factors including age, body size, and changes in prey behaviour and (or) abundance over both short and long timescales. We studied the diving behaviour of a highly sexually dimorphic odontocete cetacean, the killer whale, Orcinus orca (L., 1758), using suction cup- attached time-depth recorders (TDRs). We tested the hypotheses that dive rates (no. of dives/h greater than or equal to specific depths) of fish-eating killer whales varied between males and females, with age, between day and night, and among pods and years. Data were used from 34 TDR deployments between 1993 and 2002 in the inshore waters of southern British Columbia, Canada, and Washington, USA. Dive rates did not change with age or differ among pods or between males and females, although analyses restricted to adults showed that adult males dove deep significantly more frequently than adult females during the day. For all whales, dive rates and swim speeds were greater during the day than at night, suggesting decreased activity levels at night. Dive rates to deeper depths during the day decreased over the study, suggesting a long-term change in prey behaviour or abundance, though uncertainty regarding the diet of this population precludes determination of the cause of such changes.