By using combinations of phenotypically-distinct--but sexually-compatible--mutant strains of C. monoica (zym-1, zym-27, and nit-2) and assaying for zygote genotypes in genetically-mixed mating populations (where gametes of the two parental homothallic strains were present at similar frequencies), we have found that matings occur preferentially between cells of the same genotype. Additional support for an hypothesis of non-random mate selection was provided by using an easily-selectable genetic marker (sup-1) to verify the frequent occurrence of matings between cells of identical genotype in populations where the selectable genotype was present at very low relative frequency (10(-2) or 10(-3)) in a mixed mating population. Direct evidence for non-random mate selection was obtained by presenting nitrogen-starved cells with limiting nitrate to synchronize gametic differentiation in wild-type strains. Under these conditions, the four, eight, or 16 mitotic daughters released from the same mother sporangium often immediately established mating pairs within the group. Thus successive mitotic divisions of a single mother cell yielded progeny of opposite expressed mating-type.