Abstract: Screw fastening devices are described for use on structures such as electrical outlet boxes. According to the invention, a screw fastening device includes a support adapted to be mounted on said structure and defining two convergent opposed wall surfaces. A pair of screw-engaging members are arranged in sliding contact with the respective convergent wall surfaces. Spring means bias the members in the direction of convergence of said wall surfaces so as to normally maintain adjacent portions of said members in cooperating relationship, said adjacent portions being formed with recesses having internal grooves which together define a thread complimentary to that of said screw.