A technique based on the detection of nonlinear level-crossing resonances in finite magnetic fields has been extended to the study of high-lying electronic levels excited by stepwise excitation with two cw dye lasers. The method has been applied to the determination of |A/g J | values for four highly excited even-parity levels and two odd-parity levels in yttrium I. Using recently determined values of the magnetic hyperfine interaction constantA, Landeg J -factors have been deduced for these levels with an accuracy of about 1%. The results show significant differences from previous determinations.