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The study investigated the ecophysiological differences between populations of the Common frog(Rana temporaria L.) from different altitudes in northern England. Frog embryos in upland northern ponds experience very low temperatures during the period of embryonic development. These embryos were found to have a lower lethal limit for 50% normal development of2–8 °C, compared with3–8 °C for lowland embryos. These values are below most of the lower limiting temperature estimates previously obtained for R. temporaria. Embryos from upland ponds developed faster at low temperatures than embryos from lowland ponds. This may be an adaptation to the shorter growing season and lower temperatures at higher altitudes. Upland female frogs were found to be shorter and produced fewer eggs than lowland females. The differences found between lowland and upland frogs are discussed in terms of their potential adaptive significance.