Penguins tend to be overinsulated for life on land. A study of the Jackass penguin Spheniscus demersus was undertaken to investigate the behavioural adaptations that enable the species to cope with the exigencies of a hot, terrestrial environment. The Jackass penguin is mainly crepuscular and nocturnal at its breeding stations. The numbers of birds present on land are highest at night and lowest during the day. The majority of birds leaving a breeding colony do so just after dawn. Birds begin to return to the colony during the late afternoon. The frequency of displays is highest at dawn and from dusk to midnight. This activity cycle enables birds not engaged in incubation or chick guarding to avoid conditions promoting heat stress. Birds remaining in the colony during the day employ strategies such as burrow-nesting, body-orientation and evaporative cooling to reduce the effects of high insolation. Compared to the ambient, the microclimate of burrows is more constant; relative humidity is higher; air temperatures fluctuate less; and wind effect is negligible. Most important, birds nesting in burrows are protected from direct insolation. Egg and nest-air temperature in a burrow are relatively constant. Burrows facing east are cooler than those facing west. However this does not influence burrow-orientation, which is determined by the slope of the ground. Birds exposed to high insolation orientate with their backs to the sun. The lightly feathered areas of the body are shaded. This facilitates heat loss by convection and re-radiation from these shaded surfaces. In this position dorsal surface temperature regularly exceeds body temperature. It is suggested that heat gain from the dorsal surface is restricted by efficient insulation, primarily an adaptation to the aquatic environment. On overcast days orientation is non-directional. In the early morning when ambient temperatures are relatively low, the birds also orientate with their backs to the sun. This is interpreted as sunbathing. Body temperatures were significantly higher on a hot than on a cool day. The Jackass penguin appears to be relatively thermolabile. This would facilitate both energy conservation in water and resistance to heat stress on land. The overall thermoregulatory strategies of the Jackass penguin for life in two different environments are discussed.