The condensed bridged isoxazolines4 are rearranged on irradiation with a low-pressure mercury lamp exclusively into condensed derivatives of tetrahydropyridine5. The selectivity of the rearrangement is due to a stabilization of the biradical8 by the overlap of the radical-electrons with π-electrons of the C=C double bond and the heterocyclic ring. Quantum yields of the photorearrangement, established from the consumption of the starting materials4, were determined.