A 4.5-kg female spayed Domestic Shorthair cat presented to the Iowa State University Emergency Service (Ames, Iowa) with a 1-day history of anorexia, adipsia, weakness, and lethargy. The patient was not current on rabies or feline viral rhinotracheitis, calicivirus, and panleukopenia vaccinations and was housed outdoor and indoor. The owners noted the cat was severely ataxic the evening prior to presentation, and that she had vomited the morning of presentation.On presentation, the patient was semicomatose. Physical examination revealed severe hypothermia (34.3°C), bradycardia (137 beats/min), and eupnea (16 breaths/min). She was 10-12% dehydrated and severely hypotensive, with a systolic blood pressure of 50 mmHg and weak, thready femoral pulses. The abdomen was severely distended and symmetrically enlarged.An abdominocentesis was performed, and 1,030 ml of serosanguineous fluid was removed. Fluid analysis revealed glucose of 307 mg/dl, lactate of 9.1 mmol/l, and specific gravity of 1. Abstract. Primary tumors of serosal surfaces are uncommon in veterinary medicine. Mesothelial neoplasms can be benign or malignant, and are classified as predominantly epitheloid, mixed (biphasic), or fibrous (spindle cell, fibrosarcomatous), with fibrous mesotheliomas reported least in domestic species. A 9-year-old Domestic Shorthair cat presented on emergency with a brief history of weakness and lethargy. On presentation, the cat was semicomatose, hypothermic, and hypotensive with a markedly distended abdomen. Approximately 1 liter of serosanguineous fluid was removed via abdominocentesis. Diagnostic imaging and cytologic evaluation of fine-needle aspirates were suggestive of neoplasia, and the cat was subsequently euthanized. At necropsy, the omentum was contracted cranially into an irregular lobular mass that surrounded the stomach and proximal intestinal tract, and focally infiltrated the spleen. Both visceral and parietal peritoneal surfaces were thickened and contained off-white friable material and occasionally firm fibrous plaques. Microscopically, serosal surfaces were expanded by neoplastic spindle cells, which were often accompanied by moderate to abundant fibrous stroma. Neoplastic cells had varying degrees of immunoreactivity for cytokeratin, vimentin, desmin, and smooth muscle actin, which was consistent with the diagnosis of mesothelioma.