Malignant mesotheliomas of two Holstein cows were examined by light and electron microscopy. These tumours were derived from peritoneal surfaces and were of biphasic type composed of mesothelial lining cells and submesothelial mesenchymal cells. Ultrastructurally, the neoplastic mesothelial cells were characterized by intermediate filaments, microvilli, desmosomes, tight junctions and basal laminae. The neoplastic submesothelial cells were closely associated with collagen filaments and some cells had several resemblance to the neoplastic mesothelial cells. The ultrastructural features of the mesotheliomas are discussed in comparison with those of adenocarcinomas and fibroblastic neoplasms. In one case a malignant granulosa cell tumour involved the left ovary and its morphology was apparently different from that of the mesothelioma. Multiple primary tumours are uncommon in cattle.