Eighteen spontaneous, non-toxigenic (Tox-) mutants of Pseudomonas syringae pv. tabaci were found to cause symptomless infections in tobacco leaves. One of these mutants was selected for detailed study. Infected tissue displayed none of the physiological alterations (loss of chlorophyll, accumulation of ammonia, inactivation of glutamine synthetase) that characterized the effect of tabtoxin in infections caused by the wild-type. However, early growth in, and invasion of inoculated leaves by the Tox- mutant was not detectably different from that of the wild-type. Also, infectious cells of both cultures were transmitted with equal efficiency from inoculated to healthy seedlings in watersplash. Therefore the Tox- mutant was a non-pathogenic parasite. One important difference between the parasitic abilities of these cultures was revealed in the steady decline in numbers of the Tox- mutant in tobacco leaves from 3 days after inoculation whereas numbers of the wild-type continued to increase, or remained constant, after 3 days. The population of the wild-type in inoculated tissue grew to an apparent upper limit of 1·6 × 107 cells cm-2 leaf from inocula containing between 5 × 103 and 1·0 × 107 cells. This upper limit may have been determined by the amount of free nutrient in the apoplast. The Tox- and wild-type differed not only in toxigenicity, but also in that growth of the former was inhibited by 30 nmol ml-1 tabtoxin whereas growth of the latter was unaffected or even stimulated by as much as 3·5 µmol ml-1.