The occurrence and structure of microfilaments in epidermal cancers induced in mice by treatment with 3,4-benzpyrene were investigated with the electron microscope. With malignant change, pleomorphic, undifferentiated cells with a cortical zone of microfilaments became increasingly abundant. The microfilaments were 40 Å in diameter and occupied the cortex of the cells beneath the plasma membrane, extended into cell processes, and were situated in the cores of microvilli. At high magnification, the filamentous areas were formed by an interconnected meshwork of filaments which in favorable planes had a polygonal arrangement. When exposed to high concentrations of cytochalasin B, the microfilaments became clumped and moderately disrupted. At the same time, the processes and microvilli of the cells were blunted. The structure of these filaments and their sensitivity to cytochalasin B place them in a class of microfilaments believed to be related to cell motility. Their presence in malignant cells may be correlated with the motile, invasive properties of these cells.