There are an increasing number of studies documenting effects of global warming on the distribution and ecology of a wide variety of organisms. However, much less is known about evolutionary responses in species to increasing temperature. As a result, thermal microevolution is often ignored when making predictions about the impact of global warming on the distribution of organisms and the composition of communities. We investigated the ability of 2 large-bodied Daphnia species, D. magna and D. pulex, to genetically respond to increased temperature under semi-natural conditions. Daphnia populations were exposed for 6 mo to different temperature treatments in large outdoor mesocosms that simulated small pond systems. The selection experiment was followed by a common garden experiment in which we quantified thermal microevolution in important life history traits. Intrinsic growth rate did not respond to thermal selection. Instead, we observed an effect of selection temperature on size at maturity, indicating that Daphnia may genetically adapt to increased temperature within one growing season. In a previous laboratory study on thermal microevolution using single-species cultures of D. magna, we documented the opposite pattern with thermal microevolution in intrinsic growth rate but not in size at maturity. These observations suggest that the ecological context (semi-natural conditions compared to single species cultures) is very important in determining with which traits species adapt to global warming.