A 10-year-old, spayed female beagle was presented with a 2.5-year hist
ory of ascites. The ascitic fluid was a modified transudate with no ab
normal cells present. cardiac and hepatic functions were normal. Abdom
inal ultrasonography revealed cystic abnormalities of the peritoneal s
erosal surfaces. On exploratory laparotomy, the parietal and visceral
peritonea were covered with variably sized, thin-wailed cysts containi
ng serous fluid. Histopathological, imunohistochemical, and ultrastruc
tural analyses of the lesions were consistent with a diagnosis of cyst
ic, peritoneal mesothelioma.