Studies on the daily activity of A. lusitanicus were conducted under field and laboratory conditions. Two main phases - the activity phase and the phase of resting- have been distinguished in the daily cycle of the slug. The observed activity was of different types, such as crawling, feeding and the state of semiactivity. lt has been found that slug activity is determined by various atmospheric factors, mainly by light intensity, rainfalls and presence of dew. Slug activity is strongly influenced by their physiological 24 hr. life rhythm. Full activity was exhibited by all individuals after sunset and after the appearance of dew. They return to their shelters about three hours after sunrise. Feeding of A. lusitanicus, including periods of resting, lasts throughout the whole night, since their emergence from shelters until early morning hours. Most of the slugs start intensive feeding 2-3 hours after sunset.