The haul-out and underwater activity patterns of five Weddell seals were investigated in relation to the tide. Electronic recorders measured the seals' dive depth and internal body temperature simultaneously. Diving and temperature data were analysed with the help of time series analyses tools. We identified a half-day (semicircadian) rhythm in underwater activity and body temperature. A close correlation was found between the rhythmicity of the tide and the timing of the seals' underwater activity and oscillations of their body temperature. We propose that the tide is the principal zeitgeber for the Weddell seals' ultradian cycle of behaviour during the austral summer.