AbstractGenetic studies of several events of the life cycle ofY. lipolyticademonstrated that diploid strains were unstable and produced mitotic segregants by haploidization. A screening system was developed which enabled us to show that parasexual processes can take place in addition to the sexual life cycle. This haploidization occurred through aneuploid intermediates as was proven statistically by the deviations from the segregation pattern as well as by the segregation data of the clones.The direction of the cross, was — with respect to the resistance to 2‐deoxyglucose of A‐ or B‐strain — not important for selection of mitotic s