Corresponding double-stranded RNA segments of the related orbiviruses Wallal and Mudjinbarry produced distinctly different RNase T(1) fingerprint patterns. No extensive sequence reiteration was observed between segments of Mudjinbarry virus. Fingerprint analysis of the genome of recombinant orbiviruses confirmed segment reassortment as a mechanism of interchange of genetic information. When temperature-sensitive mutants of each virus were crossed in mixed infection, a consistent pattern of segment reassortment was correlated with generation of the wild-type phenotype. Thus, the temperature-sensitive lesion of group II Wallal serogroup mutants was mapped to segment 1. The group I mutant lesion appears to be located in segment 2.