Department of Chemistry, Bharathidasan University, Tiruchirapalli-620 024 Manuscript received 12 February 1992, revised 30 October 1992, accepted 2 December 1992 Electron-impact mass spectra of 32 isoxazolines, viz. monocyclic, bicyclic and tricyclic isoxazolines are reported. The major fragmmtation pattern for the isoxazolines with alkyl, aryl, methoxycarbonyl, (methylthio)methyl substituents at C·5 is \(\alpha\)-cleavage while there is a competition between the groups at C-5 for elimination in the case of 5,5-disubstituted isoxazolines. Bicyclic and tricyclic isoxazolines undergo ring fission to afford (isoxazole+allyl radical cation) and (isoxazole + cyclopentane radical cation) respectively, to reduce the ring strain present in the parent isoxazolines. In the case of 5-(2-metbylphenyl)- isoxazolines, the isoxazole cation is observed as the base peak arising out of CH3 loss followed by H-migration.