The function of allogeneic recognition in a marine sponge, Halichondria japonica, was examined by use of cut pieces contact assay. Individuals of this species were able to distinguish an allogeneic individual from an autogeneic one, and showed rejection reactions against allogeneic individuals. There were two types of allogeneic rejection reaction: barrier formation at the contact area to separate from allogeneic individuals and necrosis with cytotoxic reactions at the contact area. In both types of rejection reactions, mesohyl cells accumulate at the contact area at the early stages of the rejection reaction. Fusion between two pieces of allogeneic individuals was very rare, and in most of combinations of allogeneic individuals rejection reactions appeared at the contact area. Xenogeneic rejections were also observed. Halichondria japonica showed rejection reaction against individuals of Halichondria okadai, but the intensity of rejection was less than that of allogeneic rejection.