When cotton seedlings were inoculated sequentially with three cotton isolates of Agrobacterium carrying Ti plasmids (i.e., A. tumefaciens) followed 4-14 days later by R. solani, a high frequency of crown gall tumors developed at the soil line. No tumors developed when plants were inoculated with the A. tumefaciens isolates alone using a variety of wounding techniques. The synergistic interactions were observed with each of the 8 isolates of R. solani from cotton and each of the10 different cotton cultivars. Macrophomina phaseoli, Diplodia gossypii or Phoma exigua used with A. tumefaciens failed to induce galls on cotton. The results indicate that R. solani or its infections may produce unique metabolites that activate virulence genes in infectious plasmids of A. tumefaciens strains obtained from cotton roots.