Plants affected by bronze wilt in the field usually show extensive browning and necrosis of the feeder roots. The ability of Agrobacterium species to cause these symptoms was evaluated in these studies. Attempts to perform Koch’s postulates as proof of pathogenicity were complicated by survival of Agrobacterium in pasteurized soils, ready dispersal of bacteria in environment chambers, and seed transmission of Agrobacterium species. Plants inoculated with isolate 25A of Agrobacterium biovar 1, compared to uninoculated controls, had marked increases in bacterial concentrations in roots and reduction of shoot growth at 36 ºC. However, the controls had mean bacterial concentrations greater than 30 M/gm, and necrosis occurred in all treatments. Under greenhouse conditions (38 º day and 30 º night), Agrobacterium biovar 1 isolates showed distinct differences in their ability to cause root necrosis. The ability to cause necrosis was correlated with the ability to colonize roots at 37 ºC. The patterns of specificity for necrosis induction by Agrobacterium were not changed by subsequent inoculations of 4-week-old plants with Rhizoctonia solani, although the frequency of plants with necrosis, when it occurred, was increased. When isolines of ‘Acala 44’ containing different bacterial blight resistance genes (B genes) were inoculated in the greenhouse with isolate 16B of Agrobacterium biovar 1, there were marked differences among isolines in frequency and intensity of root necrosis, which were reflected in reduced plant growth and seedcotton yield. Both B ₂ and B 6 genes gave marked increases in necrosis, while B ₄ and B 7 had no effect on necrosis. Progeny from 10 plants of ‘Hartz 1215’ were uniformly resistant to race 2 of Xanthomonas campestris pv. malvacearum indicating that this cultivar contains the B ₂ gene. When seed from ‘Hartz 1215’ plants infected with isolate 34B of Agrobacterium biovar 1 were planted at 30 ºC days (14 hr) and 20 ºC nights (10 hr), some plants developed symptoms of bronze wilt. The severity of root necrosis, accompanied by stunting of the plant and chlorosis of terminal leaves, was correlated with concentrations of Agrobacterium biovar 1 in roots. Collectively, the observations indicate that certain Agrobacterium isolates can cause root necrosis especially in the presence of specific B genes and of a conductive microbial environment.