A new 'quinone methide' diterpene with a cassane skeleton was isolated from the root bark of Bobgunnia madagascariensis (DESV.) J.H. KIRKBR. et WIERSEMA (Leguminosae). Its structure was established as (4bS,8aS)-4b,5,6,7,8,8a-hexahydro-4-hydroxy-2-(2-hydroxyethyl)-1,4b,8,8-tetramethylphenathrene-3,9-dione by spectroscopic methods including single-crystal X-ray analysis. This compound showed strong antifungal properties towards human pathogenic fungi, in particular the yeast Candida albicans.