Three dsRNA virus complexes determine the expression of 3 distinct proteinaceous toxins by cells of Ustilago maydis. These toxins affect sensitive cells of the same species and related species. Based on the expression of the viral coded toxins and the dsRNA content of progeny from crosses between strains each expressing a different toxin specificity exclusion relations were noticed and defined in earlier studies. A restriction modification type mechanism operating in the exclusion process was compatible with the genetic data obtained but a second mechanism based on replication competition between the dsRNA molecules was not ruled out by these studies. The reported study provides support for the latter model based on examination of the fate of toxin expression in haploid vegetative cells resulting from heterokaryons formed between strains expressing different toxin specificities. In these vegetative cells dual specificities were detected in some instances and their segregation was examined. The results suggest that the exclusion of the toxin coding segments is related to their size and affinity to the replicase but some effects are exerted also by other dsRNA segments of the virus complex.