The Atlantic cod Gadus morhua L. persists over a wide range of thermal environments and within this range the population exhibits spatial structuring. Populations may exhibit local adaptation in their life history traits in response to their environment. Experiments are reported in which life history responses in relation to temperature were investigated in three populations that differ in their thermal regime. Post-settled wild cod from the east and west coasts of Scotland and the southern North Sea were investigated under controlled environment conditions. The populations differed in their juvenile growth rate, in their reproductive development and in their competitive interactions. Juvenile west coast cod maintained higher growth rates than east coast cod across a range of temperatures. First-time spawning west coast cod were more fecund for a given size than east coast and southern North Sea cod. Higher temperatures led to higher growth rates and a greater final body size, which enhanced fecundity. Studies of juvenile aggression in relation to population and haemoglobin genotype give insight into the mechanisms behind these life history differences. This mounting evidence of local adaptation should enable better estimates of fish population dynamics at biologically appropriate spatial scales and in response to changing environmental conditions.