To know whether or not molluscs are capable of recognizing tissue alloantigens, dorsal skin-allografts were exchanged between adult terrestrial slug, Incilaria fruhstorferi. We succeeded for the first time in orthotopic transplantation of allografts and observed chronic rejection of allografts. During the first two weeks after transplantation (WAT), in all grafts, both foreign (allo-) and self (auto-), many macrophages infiltrated from the host toward the grafts. This phenomenon is seemed to heal wounds. In the case of autografts, many macrophages observed in the grafted site until 8 weeks, whereas at 4 WAT, grafted tissues such as muscle fibers and mucous cells begun to regenerate slowly and the regeneration of these cells had been over at 20 WAT. However, in the case of allografts, regenerative phenomena were not observed, rather than muscle fibers had been actively attacked by macrophages. Numerous macrophages which phagocytosed cell debris were observed in host connective tissues during this experiment. These observations strongly suggest that an allorecognition system is present in molluscs, and in the case of terrestrial slugs dorsal skin transplantation is a useful assay system for analyses of immunological incompatibility.