SUMMARY“Phenotypic”identicalness for certain compatibility loci appears to determine whether or not 2Didymium iridisplasmodia will fuse when brought into contact. Based on almost 5,000 fusion tests, it has been firmly established that if Plasmodium X fuses with Y and Y fuses with Z, then X will also fuse with Z.Mass-spore F1plasmodia almost always display the same compatibility phenotype as their parent Plasmodium. Mass-spore-derived plasmodia are probably always heterokaryotic, containing a large number of different nuclear types.Two types of heterokaryons, other than those involving plasmodial compatibility alleles, were created. One type consisted of 2 different sets of plasmodial color factors while the other brought together 2 sets of mating type alleles.Three different homothallic isolates ofD. iridiswere studied, and all intra-isolate plasmodial combinations were compatible, but all inter-isolate ones were incompatible. It is concluded that the homothallic, as well as the heterothallic isolates, display a plasmodial compatibility system.