Optimized release conditions of hatchery-reared juvenile lobster (Homarus gammarus) can reduce loss in the field. The effect of seasonal temperature on the behavioural and physiological activity of lobsters was examined, observing the timing of moulting and locomotory activity in juvenile lobsters maintained at increasing temperatures under standardized laboratory conditions. The juveniles of 3 different year and size classes mostly moulted when a threshold temperature between 12 and 14°C was reached. In addition, the locomotory activity of lobsters showed significant responses to seasonal temperatures between 12 and 15°C in the dark-phase. The knowledge about the temperature-dependent activity of lobsters may be helpful to explain behaviour in the field and in assessing the chances of a future large scale stock enhancement programme.