We report enantioselective one-carbon ring expansion of aziridines to make azetidines as a new-to-nature activity of engineered "carbene transferase" enzymes. A laboratory-evolved variant of cytochrome P450BM3, P411-AzetS, not only exerts unparalleled stereocontrol (99:1 er) over a [1,2]-Stevens rearrangement but also overrides the inherent reactivity of aziridinium ylides, cheletropic extrusion of olefins, to perform a [1,2]-Stevens rearrangement. By controlling the fate of the highly reactive aziridinium ylide intermediates, these evolvable biocatalysts promote a transformation which cannot currently be performed using other catalyst classes.