A case of recurrent cystic peritoneal mesothelioma is reported. Ultrastructurally the tumour cells showed abundant surface microvilli, desmosomes, intracytoplasmic filaments and well-developed basal lamina. The cells demonstrated positive staining for keratin peptides, vimentin and epithelial membrane antigen and, some of them, for carcinoembryonic antigen. No staining was demonstrable for a number of endothelial markers. The findings are in accord with the proposed mesothelial origin of the neoplasm and can be of help in the differential diagnosis of other multicystic neoplasms arising in the peritoneal cavity.