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The east and south coasts of South Africa are characterised by a transition from subtropical to warm-temperate conditions. This transition in environmental temperatures may shape the physiological tolerance of ectothermic species inhabiting harsh environments, such as the intertidal zone. A subtropical (Mngazana Estuary) and a warm-temperate (Knysna Estuary) population of the truncated mangrove snail Cerithidea decollata were selected to investigate thermal tolerance (LT50) and performance (oxygen consumption) across increasing and decreasing air temperatures (at a rate of 2 degrees C h(-1)) under controlled laboratory conditions. Animals from both populations showed a considerable thermal tolerance, surviving temperatures of over 50 degrees C. The thermal performances of the two populations under temperature change showed similar trends, with individuals from the Mngazana Estuary displaying higher rates of metabolism than those from the Knysna Estuary. Both thermal tolerance and performance in increasing/decreasing temperatures suggested that this snail shows intraspecific differences in thermal physiology. We discuss the importance of including the effect of microhabitat variability and behavioural thermoregulation when investigating the effect of climatic transition on the thermal physiology of intertidal species. This would enhance our knowledge of the interactions between organisms and their environment, helping to evaluate the likelihood that a species can maintain itself in a changing landscape.