The products of alkaline hydrolysis of the host-specific phytotoxic fraction TA from A. alternata f. sp. lycopersici are identified as 1,2,3-propanetricarboxylate and either 1-amino-11,15-dimethylheptadeca-2,4,5,13,14-pentol 1 or its 7,15-dimethyl isomer. Host-specific phytotoxins, produced by more than 10 species of fungal pathogens, occur diverse structural types and include relatively low-molecular weight cyclodepsipeptides, as terpenopeptides, and unbranched long-chain polyketols. Phytotoxins of this class are toxic only to the host that is susceptible to the pathogen which produces the toxin, and if they induce nearly all the symptoms of the disease are considered to be definitive chemical probes in the study of disease susceptibility and physiological stress at the molecular level. In this and the accompanying paper we describe evidence which shows that the ninhydrin-positive hostspecific phytotoxic fraction TA, produced in culture by the tomato-specific pathogen A. alternata f. sp. lycopersici, consists of two esters of 1,2,3-propanetricarboxylic acid and the novel aminopentol. The sites of esterification are a terminal carboxyl of the acid and C13 (major component 2a) and C,, (2b) of 1.