The virulence of tryptophan auxotrophs (Trp−) of Pseudomonas solanacearum K60 was significantly lower than that of the wild type when inoculated on tobacco (cult. "Bottom Special") and tomato (cult. "Bonny Best"). These auxotrophs persisted at the site of inoculation for several weeks, but did not cause systemic infection. Auxotrophs for methionine (Met−) and leucine (Leu−) biosynthesis, however, were virulent on tobacco and weakly virulent on tomato. In tobacco leaves infiltrated with suspensions of Trp−, Met−, and Leu− mutant strains and strain K60, the population doubling times of each isolate were 16.0, 4.3, 4.2, and 2.3 h respectively. The amino acid requirement of each auxotroph was determined in vitro and compared with levels of these amino acids in xylem sap collected from decapitated plants. Tobacco sap contained sufficient concentrations of required amino acids to support 0, 53, and 54% of maximum growth in vitro of the Trp−, Met−, and Leu− mutant strains, respectively. Tryptophan, methionine, and leucine levels in tomato sap supported 0, 3, and 13% maximum growth of the respective strains in vitro. The relative concentrations of methionine and leucine in xylem sap were sufficiently different to explain the susceptibility of tobacco and resistance of tomato to these auxotrophs. The virulence of tryptophan auxotrophs was restored by genetic transformation to prototrophy and by supplying tryptophan to inoculated tobacco seedlings.