Ascospores are isolated by subjecting a sporulated culture to a temp, sufficiently high to kill the vegetable cells but not the ascospores. The spores are streaked on agar plates where they germinate and produce colonies which are transferred to slants of sporulation medium. If all colony isolates produce ascospores, sp. is homothallic; if all or nearly all of the isolates produce no spores, culture is heterothallic. Heterothallic ascospore isolates are mixed by fours on sporulation slants. Those mixtures producing ascospores contain both mating types. Mixture showing the largest % of spores is analyzed by mixing all of isolates of which it is composed in pairs. All pairs producing ascospores consist of opposite mating types. Opposite mating types from a single diploid strain are often more fruitful than parent strain, and those from different diploid strains are usually more fruitful than opposite mating types from single diploid strain. Mating types of haploid yeasts of intermediate evolutionary development may be isolated from nature. When mated, they may produce some diploid vegetative cells. These may be isolated by streaking on agar and selecting largest colonies. Mating types of physiologically different strains of a species may be mated. The diploid hybrids may be isolated by streaking on agar as soon as large number of conjugated cells are present in culture. Diploid cells produce larger colonies than haploid mating types which had not mated at time plates were streaked. Haploid hybrids can be isolated by permitting physiologically or morphologically different mating types to conjugate and produce ascospores. Vegetative cells are killed by heat and the ascospores streaked on plates. Resulting colonies may then be analyzed to learn what it has inherited from each parent. All spp. of Hansenula with Saturn-shaped ascospores are homothallic; all diploid spp. with hat-shaped ascospores are heterothallic. No hybridization between different heterothallic spp. of Hansenula was achieved, no matter how closely related spp. may be. Hansenula schneggi and Candida pelliculosa are sexually active haploid mating types of Hansenula anomala.