Most of the yeast strains of industrial interest are homothallic, which makes genetic analysis as well as improvement of their desirable properties extremely difficult. In this work, all the wine yeasts analyzed, except for two "flor" yeasts which were able to generate heterothallic meiotic products, were homozygous for the HO allele of homothallism. The genetic analysis of one of the "flor" yeast spores, which was heterothallic, indicated that the strain carried an HO allele of homothallism and that its heterothallic behaviour was due to a genetic constitution HML MAT HMR HO. This HO allele of the "flor" yeast behaved differently from other HO alleles present in the wine yeasts. It gave rise to a very retarded homothallism, with formation of selfzygotes which presented an abnormal morphology. These zygotes generated selfdiploids whose ability to sporulate was very inefficient. The lag in the expression of the homothallism allowed the isolation of hybrids between meiotic products of the "flor" yeasts and heterothallic laboratory strains with a frequency of 10(-1), as compared to that of the meiotic products of other wine yeasts, which was of 10(-4) to 10(-6). This result indicates that, among other factors which could influence the frequency of hybrid formation, the lag in the expression of homothallism could be the most important one. The use of heterothallic laboratory strains of mating type as opposed to a strain of alpha mating type, as well as the frequency of viable nucleus in the hybrid zygote after conjugation, also seemed to influence the success of hybrid formation between laboratory strains and the wine homothallic strains used.