It was recently shown by combined immunofluorescence and interference reflection microscopy that a protein named vinculin, along with alpha-actinin, is concentrated at focal adhesion plaques inside cultured normal fibroblasts [Geiger, B. (1979) Cell 18, 193-205]. These plaques are the discrete, isolated sites of strong adhesions formed between the ventral surfaces of the cells and the substrata on which they are grown. We show that after transformation of fibroblasts by Rous sarcoma virus a majority of the cells have many fewer focal adhesion plaques and now exhibit a cluster of small patches that are immunolabelled for both vinculin and alpha-actinin. Such a cluster (rosette) is located near the ventral surface of the cell, usually partly under the nucleus. The significance that these altered distributions of vinculin and alpha-actinin may have for the rounding up and loss of adherence of transformed cells is discussed.