The structure of isopauridianthoside, a compound isolated from the leaves of Pauridiantha lyallii Brem., has been determined by spectroscopic methods. This gluco-alkaloid is the C-21 epimer of pauridianthoside. The unusual configuration of the terpenic part of the molecule is discussed. Isopauridianthoside represents the first example of a naturally occurring epimer of secologanoside.