Among 120 U. maydis strains isolated in Korea 14 different strains containing specific viral dsRNA segments were analyzed for the distribution of dsRNA and the production of toxin protein. Several distinctive dsRNA patterns were identified, 9 cases of P type with typical H, M and L ds RNA and one case of non-P-type, the frequency of a specific isolate was decreased with increasing number of dsRNA segments. The presence of dsRNA had no effect on the cultural or morphological phenotype of the host. Two isolates containing P type dsRNA segments appeared to produce toxin protein (killer strains) which inhibited the growth of 4 isolates (sensitive strain) with different susceptibility. Two killer strains contain unique M dsRNA segment which may code for toxin protein. However, the presence of toxin-sensitive strains among dsRNA-free isolates was similar to that of ds RNA containing strains.