A case of solitary extramedullary plasmacytoma in the transverse colon of a 20-year-old woman is reported. This case is unusual in that the plasmacytomas are only rarely observed in the gastrointestinal tract and are especially uncommon in the colon and in young women. The lesion was composed of pleomorphic plasma cells and contained areas of extracellular amyloid deposition. Its neoplastic nature was verified by the use of immunoperoxidase studies which are positive only for lambda light chain. The subject of gastrointestinal plasmacytomas and the use of immunoperoxidase studies on plasmacytic lesions are discussed.