Large marginal deformations in string field theory

We use the level truncation scheme to obtain accurate descriptions of open bosonic string field configurations corresponding to large marginal deforma tions such as background Wilson lines. To do so, we solve for all fields as functions of the massless string field, and confirm that the effective pot ential of the massless field becomes increasingly at as the level of approx imation is increased. Surprisingly, as a result of the merging of two branc hes of the solution - one originating at zero tachyon vev and the other ori ginating at the tachyonic vacuum - this effective potential exists only for a finite range of values of the massless field. We use the D1 to D0-brane marginal transition on a circle to explore the possibility that this finite range corresponds to the infinite range of the conformal field theory para meter describing marginal deformations, but are unable to arrive at a defin itive conclusion.