Intra- and inter-specific aggression among slugs varied greatly with the species. Among pugnacious species only the mature animals displayed such behavior. Aggressiveness varied seasonally, with the most fighting occurring during summer when hot, dry weather reduced the number of shelters and availability of food. No fighting occurred in winter, when formerly aggressive conspecifics frequently aggregated. Large aggressors caused smaller conspecifics as well as other species to avoid shelters they occupied. Since slugs selected shelters closest to food, agonistic behavior improved the chances of acquiring shelter and food simultaneously.