Reflection electron microscopy of electrodeposited Co, Cu/(111) Pt ultrathin layers has revealed a heteroepitaxial and simultaneous multinuclear multilayer growth or a three-dimensional island growth, which depends on the deposition overpotential. Giant magnetoresistance has been observed in heterogeneous Co-Cu alloy films, which consist of ultrafine fcc Co-rich clusters in a nonmagnetic polycrystalline Cu matrix, prepared by electrodeposition under potential control. Such a heterogeneous Co0.2Cu0.8 alloy exhibits a large saturation magnetoresistance of more than 20% at room temperature.