Graft rejection in the earthworm, Lumbricus terrestris, is mediated by granulocytic coelomocytes (neutrophils, by light microscopy). Within 1--3 days post-transplantation of body-wall xenografts, granulocytic coelomocytes migrate to the transplant site, penetrate the graft matrix, and actively phagocytize viable muscle fibers. Destruction of muscle tissues may be augmented by lymphocytic coelomocytes (basophils, by light microscopy), but these cells participate in a minor way and do so very late in the rejection process (days 11--13). Xenografts, autografts and sham-operated controls display generalized inflammatory reactions, including coelomyocyte infiltration into the grafted tissues. In autografts, however, granulocytes only penetrate the inner longitudinal muscle layers and extensive infiltration, persistence and complete destruction never occur. Lymphocytic coelomocytes are not observed in autografts or sham-operated controls. Grafts rejection by leukocytes in an advanced invertebrate confirms that aspects of cell-mediated immunity evolved early in phylogenetic history.