The metal-insulator transition in the magnetic semiconductor Gd3-xvxS4, where v stands for vacancies, has been studied by tuning through the transition with the application of a magnetic field at low temperatures. For two samples with x=0.321 and 0.325 the transition is continuous. (T0) is linear in H-Hc which implies that (T0)(E-Ec), where Hc is a critical field and Ec is the mobility edge. This is consistent with new scaling theories of both localization and interaction effects. © 1983 The American Physical Society.