1. Seven independently isolated, methionine-requiring mutants of Pseudomonas tabaci were tested for their ability to grow in a minimal medium supplemented with a large number of compounds. The mutants responded to two methionine antimetabolites and to dimethyl propiothetin and failed to respond to cysteine, homoserine, or cystathionine. Excepting one mutant, the growth in homocysteine was very poor. The mutants displayed different levels of growth in graded concentrations of a specific compound and in equimolar concentrations of different compounds. 2. The biosynthesis of methionine in this species may differ from that in other species of bacteria and fungi. The exotoxin produced by Ps. tabaci behaves as a methionine antimetabolite, an observation which may be related to the biosynthesis of methionine in this species. 3. Biochemical mutants of Ps. tabaci displayed a pattern of virulence and avirulence for susceptible species of Nicotiana and varieties of N. tabacum. 4. The avirulence of certain mutants requiring tryptophane may be due to a previously unreported mechanism for the avirulence of nutritionally deficient pathogens: the host provides an adequate concentration of the required nutrilite, but other compounds may be needed to facilitate the uptake of the nutrilite. 5. No biochemical mutant was virulent for a species or variety that was resistant to the parental strain of the pathogen.