One gram or more of root tumors from a single sweet clover plant provided sufficient material for analysis, by polyacrylamide-gel electrophoresis, of the double-stranded RNA segments of the genome of purified wound tumor virus. This technique made it possible to select cuttings through successive vegetative propagations that resulted in a progressive increase in the proportion of virions devoid of genome segment 2 or 5. The technique can detect a segment when it is present in only 0.1% of the virions in a population. Our hypothesis for explaining the demonstrated efficacy of this method of selection provides the basis for our conclusion that segment 2 or 5 is probably entirely absent in certain selected isolates. When the reverse selection was applied to sweet clover infected with virion populations which had subnormal amounts of segment 2 or 5 and which were poorly infective on vector cell monolayers, we recovered populations with wild-type genomes which were highly infective when subsequently tested on the monolayers. Selections aimed at eliminating segment 1 never reduced the proportion of virions carrying it below ∼10%, and we conclude that at least this proportion is necessary for replication of the virus. A deletion mutation in segment 7 was different from any other in that selection resulted in the remnant of the segment completely replacing the original segemnt so that the remnant survived in a quantity equimolar to the other segments. The virus isolates devoid of segment 2 or 5 replicate well in sweet clover and are fully tumorigenic; probably neither segment bears a postulated tumorigenic gene. Isolates devoid of segment 2 did not produce the virion polypeptide of size 131,000 daltons. Segments 4 and 5 both have a molecular weight appropriate for coding the virion polypeptide of size 96,000; because isolates without segment 5 had the full complement of virion proteins it is probable that segment 4 contains the code for the protein of size 96,000. Remnants of segments that had suffered a deletion mutation did not produce any detectable polypeptide.