We carried out magnetotransport measurements and scaling analysis to r
eport on a new magnetic-field-induced metal-insulator transition in tw
o dimensions, where the separatrix between insulating and conducting s
tates appears at a critical magnetic field. In the insulating state, a
giant magnetoresistance is observed over a wide range of magnetic fie
ld. At the transition, the resistance remains nearly constant over two
decades of temperature. In the conducting state a quantum Hall platea
u is observed at h/2e2. The critical exponent is in agreement with tho
se of similar transitions in the quantum Hall regime, possibly reflect
ing a symmetry between the delocalization processes in 2D.