Freshwater crayfish are regarded as principally nocturnal, however Euastacus sulcatus is commonly observed during daylight hours, both in-stream, and wandering overland, suggesting that this species is diurnal. During the winter-summer of 2002/2003 we investigated the activity patterns of E. sulcatus using a series of continuous 24 hour observation sessions. The results of this study indicate that this species is active in all seasons, except for large crayfish (>40 mm OCL), which are rarely observed in spring and winter, but are common in summer. We conclude that E. sulcatus is a diurnal species, and that peak activity generally occurs around midday in all seasons, except in the case of small crayfish (<25 mm OCL), which become nocturnal in summer when the time of their peak activity changes from around midday, to around midnight. We suggest that this general pattern of diurnal activity may be a predator-avoidance strategy, as many carnivorous Australian mammals are nocturnal. The nocturnal activity patterns seen in small crayfish during summer may also be a predator-avoidance strategy, but the diurnal predators in this case could be the large crayfish, which are active only in the summer.