A microfilarial infection of the mammary gland of a bitch with Dirofilaria immitis was diagnosed by fine needle aspiration biopsy. The animal had had a progressively enlarging, painful, pendulous, cystic swelling, measuring 12 x 18 cm, in the second left abdominal mammary gland for four to five months. Two separate biopsies revealed microfilariae and mammary duct epithelial cells, but no inflammatory cells. A histological examination revealed atrophic and necrotic mammary lobules with dilated and cystic ducts. Solid carcinoma was also observed in a small area of the mammary mass. The interlobular connective tissue was oedematous and the lymphatic and vascular channels were dilated. Microfilariae were observed within both the small blood vessels and capillaries and in the interlobular stroma of the mammary tissue, but there was no evidence of an inflammatory response.