Tagetitoxin, a phytotoxin produced in liquid cultures of the plant pathogenic bacterium Pseudomonas syringae pv. tagetis, causes an apical chlorosis symptom when applied to stems of plants. Tagetitoxin was isolated from a liquid culture medium of the organism and purified to homogeneity by chromatography on DEAE Sephadex then LH-20 Sephadex. Chemical and spectroscopic studies allow the unambiguous assignment of the overall structure of tagetitoxin, but some stereochemical questions remain to be solved. Thus, a field desorption mass spectrum demonstrated a MW 435; other data showed that, in addition to oxygen, heteroatomic components were nitrogen in an amine, phosphorus in a phosphate ester, and sulphur. A radioactive double-labelling experiment established the phosphorus-sulphur ratio to be 1: 1. The molecular formula was deduced to be C₁₁H₁₈NO₁₃PS and a cyclic hemithioketal structure was derived from ¹H and ¹³C NMR spectra. In accord with the hemithioketal structure, tagetitoxin was observed to be labile in aqueous dilute acidic conditions yielding a product mixture that gave a positive colour reaction for thiols, a property not shared with the parent toxin. The phosphate functionality was resistant to hydrolysis by phosphatase.