Elevated temperatures and extreme climatic events, such as heat waves, can negatively affect submerged macrophytes. Here, we investigated how submerged macrophytes adapted to 3 different temperatures (ambient, ca. + 3 degrees C and ca. + 4.5 degrees C) responded to a heat wave. After 10 yr of adaptation, the shoots of 2 species of submerged macrophytes, Elodea canadensis and Potamogeton crispus, were collected from each of the 3 temperature treatments and transferred to 2 heated treatments for 1 mo. The 2 heated treatments were then exposed to a 1 mo heat wave with an additional 5 degrees C temperature increase. For P. crispus, total biomass did not differ among the plants adapted to the different temperatures or between the 2 heated treatments for the duration of the experiment. Plants adapted to the highest temperatures, however, produced a larger number of smaller turions before the heat wave and allocated less biomass to elongation before and after the heat wave. For E. canadensis, the plants adapted to higher temperatures had higher total biomass before and during the heat wave and allocated more biomass to roots and leaves during the heat wave. Most indicators (e.g. length and biomass) of macrophyte performance measured during the experiment did not differ between the 2 heated treatments. In summary, after the 10 yr adaptation to higher temperatures, the submerged macrophytes showed adaptive changes in growth and asexual reproduction and responded in a complex way to the heat wave depending on species, growth status and adaptation temperature.