We find local minimizers of the two-dimensional Ginzburg-Landau functionals
depending on a large parameter k, which describe the behavior of a superco
nductor in a prescribed exterior magnetic field h(ex). We prove an estimate
on the critical value H-c1 of h(ex)(k), corresponding to the first phase-t
ransition in which vortices appear in the superconductor; and we locate the
se vortices.