Broadly distributed plants have to cope with dramatic differences across latitude in the prevailing environmental temperature. We investigated the effect of water temperature on plant morphology, biomass accumulation and oxygen-exchange for five clones of the submerged aquatic macrophyte Potamogeton pectinatus L., originating from 42 to 68°N. 2 We tested whether P. pectinatus clones show local adaptation to the prevailing environmental temperatures (in which case high-latitude clones would perform better at lower temperatures and vice versa). Alternatively, pronounced phenotypic plasticity in the response to temperature could enable individual clones to perform well over a wide range of environmental temperatures (i.e. high degree of thermal tolerance). 3 The overall pattern of thermal response was similar for all clones. In addition, we detected acclimative phenotypic plasticity in both physiological and morphological plant parameters. The optimum temperatures for gross or for net photosynthesis did not vary with growth temperature, but morphological acclimation partly compensated for the loss of photosynthetic capacity at higher temperatures, enabling comparable rates of ambient gross photosynthesis. Respiratory reactions also showed some d 4 As a result of the combined effects of changes in morphology and physiology, all clones produced similar amounts of plant biomass over a relatively wide range of water temperatures. We therefore conclude that P. pectinatus is thermally tolerant and not locally adapted.