  We examine the dynamics of a self-gravitating string in the scalar-tensor
theories of gravitation by considering a thin tube of matter to describe it.
For a class of solutions, we obtain in the generic case that the extrinsic
curvature of the world sheet of the central line is null in the limit where the
radius of the string tends to zero. However, if we impose a specific constraint
on the behaviour of the solution then we find that only the mean curvature of
the world sheet of the central line vanishes which is just the Nambu-Goto
dynamics. This analysis can include the massless dilatonic theories of gravity.
