A 7-years-old spayed, female Siberian Husky dog was diagnosed with malignant poorly differentiated intestinal mast cell tumour. The dog had previous history of colonic perforation which might have been associated with this malignant disease. The dog was presented with ascites which is uncommon sign of this disease. The disease was tentatively diagnosed by cytologic examination of the abdominal fluid and later confirmed histologically and by immunostaining.