A simple, quick technique for isolating sets of a, α, a/α, a/a and α/α isogenic strains of the yeast, Saccharomyces cerevisiae is described. Isogenic a/α diploids arise in haploid populations by a rare heterothallic switch of mating type followed by mating of the switched cell with one of the other cells in the population. Sucrose density gradient centrifugation was used to select for large elliptical diploid cells in a population of smaller haploid cells, since diploid cells are larger and more oval than haploid cells. From an a/α diploid strain obtained in this manner, ala and α/α cells were isolated by selecting for mating ability using a procedure similar to marker recovery. Finally isogenic a and α haploids were simply obtained by sporulation and dissection of an a/α isogenic diploid strain.