Nucleic acid probes prepared directly from bluetongue virus (BTV) genomic double-stranded RNA (dsRNA) have been used to identify the functionally equivalent genome segments from six distinct isolates of BTV after their separation in both agarose and polyacrylamide gel electrophoresis systems. Variations in the rate, and in one case the order, of migration of the equivalent genome segments from different viruses was detected in the polyacrylamide gel system. However, the genomic dsRNA profiles of eleven BTV isolates were found to be identical when analysed by agarose gel electrophoresis. Functionally equivalent genome segments from the six viruses that were analysed were found to migrate in identical relative positions in this gel system. From these data we propose a modified version of the protein coding assignments published for BTV 1 South Africa (Mertens et al., 1984) in which the identification of the genome segments would be based upon their order of migration in the agarose rather than the polyacrylamide gel system. The modified coding assignments, unlike the original assignments, would be applicable to all of those viruses analysed and appear likely to be valid for all normal BTV isolates.