The distribution of desmin and fibronectin in the chick embryo gonad in and after sexual differentiation was investigated immunohistochemically. In the undifferentiated gonad (stage 27), fibronectin was detected in the dorsal mesentery, basement membrane and part of the parenchyma, while desmin was not recognized. After sexual differentiation (stages 31 to 34), desmin-positive cells gradually increased in number with the progress of development. They were obviously seen in developing interstitial regions between testicular cords. However, fibronectin was hardly observed at the same stages. At stage 37, desmin-positive cells were present only in a part of the Leydig cells and myoid cells, while fibronectin was discontinuous in the basement membrane surrounding testicular cords. In addition, the expressions of desmin and fibronectin were simultaneously and adjacently recognized. These results suggest that myoid cells may produce fibronectin protein and that fibronectin mainly contributes to the formation of testicular cord basement membrane in chick embryos. Thus, the cell-to-cell and cell-to-matrix interactions may be important in testicular morphogenesis of chick embryos.