SUMMARY - This report describes thè anatomohistopatological, ultrastructural and immunohistochemical findings in a 7-year-old mongrel bitch affected with pericardial mesothelioma. The bitch during 8 month had clinical signs suggesting pericardial effusion. Histological examination of thè biopsy specimen revealed nests of suspected mesothelial neoplastic cells. Given its unfavourable prognosisi thè dog was euthanatìzed. At necropsy, thè parietal and visceral surface of thè pericardium were covered with irregular brown-yellow,velvety nodules and plaques, Neoplastic nodules and plaques were also present in thè pleura. Histologically, diffuse cuboidal epithelioid cells cove-l ring papìllary projections on a spindle celi fibrovascular core were found. The ultrastructural and immunohistochemical findings were characteristic of mesothelial cells.