Administration of sodium [2-2H3]acetate to the crysophyte Ochromonas malhamensis yielded [2H22]poriferasterol, from which, by chemical degradation, the C(24)-C(29) fragment of the side chain was obtained as (2S,3R)-2-[2H2]methyl[1,1,1,2-2H4]pentan-3-ol. 2H N.m.r. analysis of this deuteriated compound shows that the 1,2-hydride migration from C-24 to C-25 engages the si-face of the Δ24-sterol double-bond, and suggests that S-adenosylmethionine approaches the same double bond from the re- face.