Behavioural activity, and core and surface temperatures of 4 unrestrained Galapagos fur seals were recorded in the natural habitat during their first weeks of life. Climatic variables were registered simultaneously. Pup behaviours were divided into bouts of resting (55% of total time), sucking (23%) and other activities (22%). Pups maintained a constant body temperature from their first day. Core temperature (T e ) was 37.7° C±0.3° C (x ± SD) over 39 pup-days and 8 pup-nights. Skin temperature was correlated with T c , but flipper temperature was not. No daily T c rhythm was detected. Microclimate data were used to calculate operative temperature T e . Environmental temperatures can be very high, with T e above T c 6-9 h a day for animals exposed to the sun, but below it in the shade. T c is about 22° C at night. Pups avoid overheating mainly by withdrawing into the shade and reducing activity to a minimum during the hot hours of the day. Sun-exposed pups could be active at any time during the day if they had access to water, which was usually around high tide.