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A new fusicoccin wilting toxin termed fusicoccin H has been isolated from culture filtrates of the plant pathogen fungus Fusicoccum amygdali Del. The constitution of fusicoccin H has been established by chemical and spectroscopic methods. The stereochemistry of the fusicoccin H aglycone has been proved by chemical correlation with the known fusicoccin aglycone. This is the first member of the fusicoccin group to be isolated which has no isopentenyl unit on the sugar moiety and it is also devoid of the methoxyl group on the aglycone. Attempts to demethylate fusicoccin derivatives to generate compounds corresponding to fusicoccin H led only to rearranged products. Fusicoccin H has been labelled specifically with tritium at C-8. Feeding experiments with labelled fusicoccin H indicate that it can act as a precursor of fusicoccin and thus provide the first direct evidence that fusicoccin is probably a diterpene. The appendix describes a synthesis of all transgeranylgeraniol specifically labelled with tritium at C-9. Feeding experiments indicated that this precursor was incorporated into fusicoccin but degradation has shown that the label could have been randomised.