Mesothelial cells are believed to be the progenitor cells for malignant mesothelioma, a tumor associated with exposure to asbestos and other mineral fibers. Little is known regarding fibronectin (Fn) function in mesothelial and mesothelioma cells. Fn RNA, protein levels, and localization were assessed in secondary cultures and later passages of spontaneously immortalized rat pleural mesothelial (NRM) cells and in neoplastic cell lines derived from asbestos-induced mesotheliomas. NRM cells expressed similar levels of Fn RNA regardless of passage number or cell density, as determined by Northern blotting and ribonuclease protection assays. Western blotting showed that Fn protein was both secreted by NRM cells and associated with cell lysates. Immunofluorescent confocal laser scanning microscopy demonstrated that secondary cultures of NRM cells assembled Fn into abundant homogeneous fibrillar arrays organized primarily between cells, whereas later passages of NRM cells displayed abundant but less homogeneous Fn organization. Fn RNA and protein levels in neoplastic mesothelial cells were slightly less or similar to levels in NRM cells. Organization of Fn in neoplastic cells was heterogeneous compared with secondary cultures of NRM cells, but Fn fibril formation was still apparent. F-actin microfilaments were organized in both NRM and neoplastic cells; however, actin stress fibers were maintained in neoplastic cells, whereas NRM cells displayed dense actin peripheral bands at high density. The maintenance of organized Fn and actin in mesothelioma cells is surprising and may contribute to the localized growth and invasive properties of these tumors.