The induction of forward mutations (resistance to canavanine) by heavy ion bombardment was investigated in wild type haploid yeast Saccharomyces cerevisiae. Accelerated ions of argon, titanium, nickel, krypton, xenon, lead and uranium with specific energies between 1.7 and 9.25 MeV/u were obtained from the UNILAC machine at the Gesellschaft für Schwerionenforschung, Darmstadt/Germany. LET-values ranged from 1200 to about 15 000 keV/microns. There was no unequivocal dependence of mutation induction cross section on either LET or Z*2/beta 2, but also a prominent influence of ion specific energy. This is explained by the action of long-ranging delta-electrons.