The presence of a well-developed tight junction system in a case of diffuse, papillary, malignant mesothelioma of the peritoneum is documented using the freeze-fracture methodology. The mean number of strands within the tight junction network was determined to be 3.83 i 0.16. In addition, the tight junctions were specifically located in regions of the plasma membrane adjacent to recognizable microvilli. Gap junctions and desmosomes were found in close association with the occluding complexes; however, they were also seen in areas of the plasma membrane quite distinct from those that exhibit tight junction differentiation. The possible significance of the presence of tight junctions in neoplastic epithelial cells is also discussed.