The genome of the 3 Ustilago maydis viruses, known to be associated with the "killer phenomenon", is segmented. The distribution of the viral functions on different segments has been partly resolved. The reported comparative study, of a series of mutants with deleted segments of the genome and of hybrid variants containing segments of the related viruses, provides additional information on the location of various functions and on the interrelations among the different segments. The data indicates that the functions related to the maintenance of the viral information are located on the H segments. Among these segments some function overlap exists. The maintenance of the M and L segments is dependent on the functions located on the H segments. The functions related to the killer phenotype are distributed on the M2 and L1 segments. These functions include the determination of the killer specificity. The H segments play no role in the determination of the killer specificity and the specificity is determined by the M2 segment. The L segment is essential for the expression of the killer phenotype but it is suggested that the information for the toxin resides in the M2 segment.