A major fungitoxin in the root of Pachira aquatica (Bombacaceae) was isolat
ed and identified as isohemigossypolone. Quantitative analysis of isohemigo
ssypolone in different parts of P. aquatica revealed that the outer bark of
the swollen trunk accumulated a large amount of the fungitoxin. The concen
tration of the substance was more than 1 mg/g fresh weight in the outer bar
k tissues. Pure isohemigossypolone showed clear antifungal activity against
Pythium ultimum at a minimum dose of 10 mu g/disk, and it may function as
a defensive chemical to protect storage tissues of P. aquatica.