An antifungal saponin was isolated from the bottom cut of white asparagus (Asparagus officinalis L), which is unusable in food processing. The structure of the saponin was identified as 3‐O‐[{α‐L‐rhamnopyranosyl (1→2)} {α‐L‐rhamnopyranosyl (1→4)}‐β‐D‐glucopyranosyl] (25S) spirost‐5‐ene‐3β‐ol from chemical and spectral data. This saponin was shown to be identical with collettinside III from Dioscorea collettii, and to inhibit the growth of some kinds of fungi at μg ml−1 levels.