ABSTRACT: The impact of metacercaria infestation on the metabolic activity, thermal tolerance and freezing tolerance of Corophium volutator (Pallas, 1766) was investigated. Metabolic activity was measured by direct microcalorimetry at differenttemperatures and oxygen conditions. In 2 cases only, at 15°C/50% O₂ and 10°C/100% O₂, a positive correlation between infestation intensity and metabolic heat loss was found. At all temperatures examined, hypoxia down to 35% oxygensaturation did not have a significant effect on metabolic heat loss. With rising temperature, enhanced locomotory activity of C. volutator was observed during measurements, but an effect of infestation intensity was not found. According to theresults of the thermal tolerance experiments, metacercaria infestation at studied intensities does not compromise infested hosts. In the freezing tolerance experiments, heavily infested C. volutator specimens survived better than specimens withlower infestation intensities. The results indicate a good adaptation of the parasite, because survival of the second intermediate host until consumed by the final host is essential for the completion of the life cycle of digenetic trematodes. It isconcluded that under normal field conditions, changes of metabolic activity or reduced tolerance towards thermal stress and freezing as a result of metacercaria infestation should not be responsible for mass mortality events observed in the field. Therole of the developmental stage of the metacercaria and the possibly more severe impact caused by not yet or only recently encysted metacercariae, compared to metacercariae with multi-layered cyst walls, is discussed.