A mutant of the Prague strain of Rous sarcoma virus (RSV) has two deletions near the 5'-terminus of its src gene. The mutant (dl 5) induced fusiform morphological transformation in chick embryo fibroblasts (CEF) in contrast to the round type morphological transformation induced by wild type (wt) RSV. The partially deleted src of dl5 was shown to code for a 52K protein which has tyrosine kinase activity as high as that of p60src. In dl 5-transformed cells, a 36K protein was heavily phosphorylated, while two proteins (MW = 55K and 57K) were dephosphorylated to the same extent as compared with cells transformed by wt RSV. The relative abundance of phosphotyrosine in vinculin was almost the same in both dl 5- and wt RSV-transformed cells after extended passage of dl 5-transformed cells without marked changes in their morphology. On the other hand, the amounts of vinculin and fibronectin in dl 5-transformed cells were twice those in wt RSV-transformed cells, although less than the amounts in uninfected cells. These results suggest that events leading to cellular morphological transformation are neither directly correlated with phosphorylation of the 36K protein, with dephosphorylation of the 55K and 57K proteins, nor with the level of phosphorylation of tyrosine residues in vinculin, but rather are correlated with the amount of vinculin and fibronectin.