Tabtoxin, also known as wildfire toxin, is a simple monobactam biotoxin produced by Pseudomonas syringae. It is the precursor to the antibiotic tabtoxinine β-lactam. Tabtoxin is a monocyclic β-lactam produced by P. syringae pv. tabaci, coronafaciens, and garcae. Pseudomonas syringae pv. tabaci, the causal agent of the wildfire of tobacco, produces the phytotoxin tabtoxin. tabtoxin-producing bacterium, P. syringae BR2, causes a disease of bean similar to tobacco wildfire. This organism is closely related to P. syringae pv. tabaci but cannot be classified in the pathovar tabaci because it is not pathogenic on tobacco. Tabtoxin has been shown to be a dipeptide precursor that must undergo hydrolysis by a peptidase to yield the biologically active form, tabtoxinine-p-lactam (TβL). Tabtoxin is required by BR2(R) for both chlorosis and lesion formation on bean. All mutations that affected tabtoxin production, whether spon- taneous deletion or transposon induced, also affected lesion formation, and in all cases, restoration of tabtoxin production also restored pathogenic symptoms. Other factors may be required for BR2 to be pathogenic on bean, but apparently these are in addition to tabtoxin production.