Strains of Dictyostelium discoideum were paired in various combinations. Certain pairings resulted in the production of macrocysts, which are thought to be the sexual stage in the cellular slime molds. One selfcompatible strain was found. Other strains produced macrocysts only when paired with an alternate mating type. Two strains were found to form macrycysts when paired with either mating type but not alone or with each other. Several strains did not form macrocysts under any circumstances. Strains were graded according to their mating competency based on the number of macrocysts produced relative to the aggregated myxamoebae observed in the pairings.