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A population of the Common frog (Rana temporaria L.) which was self-maintaining for food, subject to the minimum of interference and derived from one batch of spawn, was kept for nearly seven years in an enclosure of 65 m2 in which cover was provided by a pile of humified plant litter and by herbaceous vegetation. The seasonal pattern of activity, and observations on feeding and other aspects of behaviour are described. Hibernation took place in the plant litter when the only water available was in concrete pools up to 18 cm in depth, but there was evidence of preference for an underwater site for hibernation after a well vegetated concrete pond 50 cm deep had been provided. Individuals which could be readily observed were found to adopt fixed resting places during the summer. Breeding first occurred at the age of three years. Censuses made possible by aggregation for this purpose and also, in the last winter of the study for hibernation under water, indicated a population of 26 individuals at the age of three years, falling to 16 at the age of six years. A second cohort raised a year after the main population and under the same environmental conditions was eliminated: the reason for the failure of any of its members to survive is discussed. A provisional estimate is made of the population metabolism during the latter part of the study and a comparison is made with that of small homeotherms. Attention is drawn to the advantages of the low metabolic rate permitted by poikilothermy for small insectivorous land vertebrates.