Soft magnetic fibers, 20 to 50 mu m in diameter, have been cast by melt extraction. A giant magnetoimpedance effect (GMI) of about 60% was observed in a (NiCo)(70)FeSiBMn fiber driven by a rf current. The magnetic field and frequency responses of fibers depend on the density and the frequency of the drive current. The GMI effect at saturation decreases from 70% at 3 mu A/mu m(2) to 55% at 30 mu A/mu m(2). The saturation field increases with an increase in frequency, and is larger for lower current densities. These fibers can be used as sensing elements for a new generation of magnetic field sensors.