[reaction: see text] The stereoselectivity of hydroxylation of alkyltetrahydropyran-2-ols (or their biological equivalents) in the formation of stereoisomers of 2,8-dimethyl-1,7-dioxaspiro[5.5]undecanes in male Bactrocera cucumis has been investigated. Racemic, (6R)-, and (6S)-6-methyl-2-[5-(2)H(1)]-n-pentyltetrahydropyran-2-ol was administered under an [(18)O(2)]-enriched atmosphere. The stereochemistry and isotopic composition of generated spiroacetals were monitored by combined enantioselective GC-MS. The monooxygenase(s) strongly prefers the (6S)-substrate and furnishes predominantly the (S)-alcohol and then the (2S,6R,8S)-2,8-dimethyl-1,7-dioxaspiro[5.5]undecane. The (2S,6S,8R) and (2R,6S,8S) (E,Z)-isomers appear to be derived in vivo predominantly from (R)-hydroxylation of the (6S)-tetrahydropyranol.