Five phenolic constituents have been identified in Yucca schidigera bark, and their structures were established by spectral (FABMS and NMR) experiments. These included two known stilbenes, trans-3,4',5-trihydroxystilbene (resveratrol) and trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene, as well as three novel compounds, yuccaols A, B, and C, with spiro-structures rarely occurring in the plant kingdom. It is suggested that yuccaols A-C are biosynthethized via attachment of a stilbenic derivative to the carbocationic intermediate of the oxidative flavanone-flavonol conversion.