Cycloaddition of aldehyde and ketone oximes are shown to give mixtures of all possible isoxazolidine regioisomers and stereoisomers. The products are 2 : 1 adducts with the second molecule of the di-polarophile attached to the isoxazolidine N-atom. The stereochemistry of the major isomer from benzaldehyde oxime and acrylonitrile was established by an X-ray crystal structure analysis. The cycloaddition is shown to be weakly catalysed by 2,4-dinitrophenol and to proceed best in acetonitrile. More polar solvents slightly favour the 5-isoxazolidine regioisomer. The mechanism of the reaction is discussed.