Eight strains of sake yeast exhibited inherent-resistance to 100 mug/ml of Geneticin (G418). Fourteen wine yeasts and 1 shochu yeast (Saccharomyces cerevisiae) and 1 miso yeast (Zygosaccharomyces rouxii) were inherent G418-sensitive. The petites converted from inherent G418-resistants by treatment with ethidium bromide retained G418-resistance (rho- G418R), and thus were hybridized by electrofusion with the wine yeast W3 (rho+ G418S, wild type). A lag phase of 12-18 h was required prior to administration of the drug in glycerol medium when selecting G418-resistant hybridization products. Colonies were formed in the regeneration medium at a frequency of about 1 x 10(-5) per used protoplasts. No growth of any parental strain (10(6)-10(7) protoplasts) separately subjected to electrofusion and regeneration was observed. The hybridization products were G418-resistant "grande" strains (rho+ G418R) in which the genetic traits of parental strains had been complemented. Uninucleate cells (DAPI staining) of the hybridization products showed CHEF electrophoretic karyotypes similar to that of wine yeast, but possessed a single chromosome (approx. 320 kb) presumably from sake yeast.