It has been reported that allorecognition is present in some invertebrate species, but it has not been confirmed in molluscs. Thus, we orthotopically transplanted dorsal skin from the terrestrial slug, Incilaria fruhstorferi, and continued to observe the grafts and their beds light and electron microscopically until 20 weeks after transplantation. During the initial 2-4 weeks, numerous macrophages were induced from the graft bed of the host towards donor tissue in both auto- and allografts. The cell debris of donor tissue was phagocytosed by the macrophages. About 8 weeks after transplantation, muscle fibers began to regenerate in the autografts, but not in the allografts. Mucous cells regenerated within 4 months only in the autograft, while in the allograft no significant regeneration of them was observed and many macrophages which phagocytosed the cell debris of the graft were observed in the connective tissue even 20 weeks after transplantation. These results show that an allorecognition system is present in mollusca as in other invertebrates such as echinodermata, annelida and that an protochordata and allograft is chronically rejected for a long term.