Little is known about intraspecific variation in fitness performance in response to thermal stress among natural populations and how this relates to evolutionary aspects of species ecology. In this study, population growth rate (PGR; a composite fitness measure) varied among five natural Chironomus riparius populations sampled across a climatic gradient when subjected to three temperature treatments reflecting the typical range of summer habitat temperatures (20, 24 and 28 °C). The variation could be explained by a complex model including effects of genetic drift, genetic diversity and adaptation to average temperature during the warmest month, in addition to experimental temperature. All populations suffered a decrease in PGR from 20 to 28 °C and ΔPGR was significantly correlated with the respective average habitat temperature in the warmest month-populations from warmer areas showing lower ΔPGR. This implies that long-term exposure to higher temperatures in the warmest month (the key reproductive period for C. riparius) is likely to be a key selective force influencing fitness at higher temperatures. A comparison of phenotypic divergence and neutral genetic differentiation revealed that one phenotypic trait--the number of fertile egg masses per female--appeared to be under positive selection in some populations. Our findings support a role for response to temperature selection along a climatic gradient and suggest population history is a key determinant of intraspecific fitness variation. We stress the importance of integrating different types of data (climatic, experimental, genetic) in order to understand the effects of global climate change on biodiversity.