SUMMARYMultiple myxamoebal clone crosses of the myxomyceteDidymium iridisresult in the production of a single Plasmodium even when it is theoretically possible to produce a number of plasmodia of different incompatibility phenotypes. The dominant survivor can be predicted beforehand by determining the value of the incompatible cytotoxicity potential of all the possible phenotypes since the dominant Plasmodium will have a higher relative toxicity value. This elimination of different plasmodial-incompatibility phenotypes from culture is viewed as one more manifestation of a self-nonself recognition system.