Radio-telemetry was used to investigate locomotor activity rhythms in a field population of the freshwater crab Potamon fluviatile. Fourteen adult specimens were radio-tracked almost continuously over a 13-day period along a hill stream in Tuscany, Italy. A rough nychthemeral movement pattern was found in some animals of both sexes. None of the crabs showed a constant activity on different days, nor did they follow any periodical pattern. Although average displacement amounted to about 5m/d, activity was concentrated over several days at a time, followed randomly by periods of almost total inactivity. On average, only about 3/4 of radio-tagged crabs were active every day and only a small fraction active at the same time. The crabs spent about 60% (females) or 70% (males) of their time out of the water, but there was no regular pattern in land-water preference. The random high variation in locomotor activity on different days seems to be a constant pattern in all decapods that have been studied so far, using individual marking techniques.