We have precisely localized virulent (vir) genes of the hairy root-inducing plasmid pRiA4b on the basis of sequence similarity with the tumor-inducing plasmid pTiA6NC, and shown that the overall organizations of vir genes in both plasmids are fairly analogous, although sizes and spacer lengths in some genes differ from each other. Among the vir genes thus mapped, the virC and virD loci were characterized in detail. Transposon insertions in virD led to loss of tumorigenicity on Kalanchoe stems and carrot discs, and one within virC exhibited an attenuated pathogenicity. The avirulent phenotype of the virD2 strain among these mutants was due to the lack of ability to recombine T-DNA border repeats in Agrobacterium cells. The nucleotide sequence of most parts of the virCD loci were similar in both plasmids. The virCD genes of these two plasmids, therefore, seem comparable both functionally and structurally. Phylogeny of pRi and pTi has also been discussed from the sequence data.