Myofibroblasts were observed at sites of connective tissue proliferation in 17 cases of feline mammary carcinoma (15 adenocarcinomas and 2 solid carcinomas). The cytoplasm of these cells appeared bright red with Mallory-azan stain and dark blue wth levanol fast cyanin 5RN, indicating the presence of contractile protein. The appearance of these cells by electron microscopy agreed with other reports on myofibroblasts: indented nuclei, remarkable accumulation of microfilaments in the cytoplasm, organization of desmosomes and hemidesmosomes on the cell surface and the formation of a basement membrane-like structure. Such myofibroblasts were found in both the interstitial connective tissue and fibrous capsule, and in some cases they showed widespread proliferation in the stroma having a myomatous appearance. On the other hand, they could not be detected at the sites of invasion in some cases. These morphological features suggest that the organization and participation of myofibroblasts may be of particular significance, i.e., reaction to tumor cells or transformation together with tumor cells, in feline mammary carcinoma.