Taqetitoxin is a phytotoxin produced in liquid cultures (MITCHELL and DURBIN, 1981) of the plant pathogenic bacterium Pseudcmonas syringes pv. tagetis (Helmere 1955) Young, Dye & Wilkie 1978). Plants when treated with tagetitoxin, or sometimes when infected by P. syringes pv. tagetis, develop an apical chloroeis (STYER et e1 ., 1980 ; TRIL~OLI et a1 ., 1978), an effect thought to be the result of disruption of the normal develo~ent of chloroplasta (LU1QiN5 and DURBIN, 1981) . Tagetitoxin has been isolated from a liquid culture medium. Following purification by ion exchange chromatography on DEAR Sephadex and partition chromatography on LH20 Sephadex it was obtained as a noncryatalline, glassy residue. It was highly water soluble but had little apparent solubility in non-aqueous solvents . Thin layer chromatography and thin layer electrophoresis procedures indicated a homogeneous preparation. We report data which allow the proposal of the novel structure 1 for tagetitoxin.