The parasitic interaction between brown trout (Salmo trutta) and the freshwater pearl mussel (Margaritifera margaritifera) is complex where the two species through mutual adaptations are maximising their fitness in an evolutionary arms race. While M. margaritifera is obligatorily dependent on salmonids for its reproduction, a high grade glochidia infection can result in significant physiological costs for the host in the form of impaired respiratory capacity, reduced swimming performance and inhibited foraging. In response to this parasitic burden ectothermic organisms can utilize behavioural thermoregulation as a defence mechanism to optimize their physiological state. To determine if infected S. trutta can alter their thermal preference a study was conducted using a temperature preference chamber with a controlled temperature gradient. The aim was to isolate the importance of temperature for fish behaviour independent of external factors such as predation or flow. The results revealed a significant difference in temperature selection where infected individuals exhibited behavioural fever by seeking out warmer water compared to uninfected individuals. This indicates that at a high infection rate there may be a threshold value where the burden from the glochidia infection becomes more costly for the individual than the higher metabolic expense of remaining in warmer water. To accelerate glochidia development through heat may further increase the risk of trophic mismatch where juvenile mussels are forced to conclude their parasitic phase before optimal feeding conditions prevail in the stream. To strengthen the two species survival and dispersal capability it is vital to preserve thermal refugia and shaded riparian zones as well as to prioritize connectivity and habitat heterogeneity in stream restoration.