SUMMARYPlasmodial (somatic) incompatibility reactions in the Ven 1 isolate of the myxomycetePhysarum cinereumare controlled by two unlinked loci displaying simple dominance. Phenotypically dissimilar plasmodia undergo temporary fusions which are quickly terminated by a cytotoxic reaction, whereas phenotypically similar ones undergo fusions which are not followed by such reactions. Each locus allows a killed zone (area of cytotoxicity) of characteristic size when acting alone, but when the two loci act together they may reduce the size of the zone with a genetically additive effect. These reactions are generally unidirectional for each locus, with killing taking place in the recessive phenotype.