ABSTRACT: Heart rate data loggers were implanted into king penguins Aptenodytes patagonicus undertaking foraging trips at sea during three austral summers. Data were obtained from a total of 20 king penguins. Our aim was to investigate variations in mean heart rate over the dive cycle (i.e. dive plus the subsequent surface period) with changes in diving behaviour, at the scale of the individual foraging dive. From these heart rate values, energetic costs were estimated in terms of rate of oxygen consumption. Shorter dive durations and longer surface durations were associated with statistically significantly higher mean heart rates, and hence energetic costs, over the dive cycle. A decrease in the duration of the bottom period of dives was also associated with an increase in heart rate and energetic costs. In contrast to model predictions that power requirements of swimming in penguins increase rapidly with increasing speed, in the main, the number of wiggles (i.e. intensive prey pursuits lasting several seconds) in a dive did not affect mean heart rate. Furthermore, the shape of the dive did not affect mean heart rate. Diving behaviour and mean heart rates over the dive cycle of birds during 2 austral summers was compared. While the birds in the earlier austral summer undertook considerably longer foraging trips than did the birds in the latter summer, mean mass gain while at sea was similar. Birds during the latter summer exhibited statistically significantly longer dive durations and bottom durations and dived to statistically significantly shallower depths than birds during the earlier summer. However, this did not translate into a statistically significantly lower mean heart rate over the dive cycle in these birds. This suggests that the differences in dive time budgeting between the two summers had only a relatively subtle effect on mean heart rate. In turn the relationship between diving behaviour and foraging success in king penguins may be more obvious than that between diving behaviour and mean heart rate over the dive cycle.