We have examined the hypothesis that fibroblasts are composed of different cell types which can be distinguished immunochemically and which possibly display different functional activities. For this purpose we have investigated fibroblasts isolated and cultured from rat skeletal muscle, dermis, subcutaneous tissue, and lung. At the 5th to 10th passage, such cells had a proportion of α-smooth muscle (SM) actin positive cells stable per each cell type. A 3-day culture was selected as a standard time for staining and evaluation; We have studied parameters characterizing cell shape and spreading and shown that there is a correlation between the presence of α-SM actin on fibroblasts and their shape in particular the presence of "capes", defined as evagination longer than 15mm and with a width at their base less than 10mm. We have also studied other features of fibroblastic cells such as the presence of focal contacts and the organization of fibronectin and always found differences between fibroblastic cells containing α-SM actin and those not containing this protein. Our work reinforces the assumption that fibroblasts are an heterogeneous cell population in which each cell phenotype is probably exerting specific and specialized functions.