The occurrence of autotomy was investigated in five terrestrial slug species. Half of the Dcroceras reticulatum and 80 per cent of the Limax tenellus slugs which were stimulated mechanically in the tail, autotomized the tail. Anon fasciatus, Anon subfuscus and Limax cinereoniger did not show any signs of being able to autotomize. The autotomized tail of D. rcticulatum and L. tenellus was small compared to the total body weight and also to the weight of many predaceous beetles. D. reticulatum and L. tenellus escaped in a few seconds after the beginning of mechanical stimulation; the reaction time was significantly shorter than that used by species which did not autotomize their tail. Comparison between A. fasciatus and D. reticulatum also showed that D. reticulatum sought for refuges significantly more effectively than A. fasciatus. The exposure of D. reticulatum to the beetle Ptero-stichus mger indicated that autotomy can stop a beetle long enough to let the slug escape. It seems that autotomy has evolved more frequently in those species which rely to a great extent on escaping and hiding when defending themselves, than in species whose physiology and/or environment do not enable them to hide quickly after an attack.