ABSTRACT Locomotor activity of the aquatic/land crab Potamon potamios Olivier was studied at different temperatures and with reference to water balance conditions. Large crabs were diurnal more often than medium-sized ones. Both groups differed also in their activity pattern at different temperatures. Large crabs showed signs of a temperature adaptation as indicated by a drop in activity at high temperatures. Water loss up to 30°C was minimal and could not be a factor affecting the behavior of the crabs. At higher temperatures water loss increased and could affect the activity pattern.