Polyethylene single crystal lamellae and lamellar stacks were deformed on an extensible substrate after exposure to doses of $\gamma $ -irradiation from 0 to 210 Mrad in vacuo, and examined in the electron microscope. Unirradiated crystals deformed affinely with the substrate by a variety of crystallographic processes including slip, twinning and a strain-induced phase change. Irradiation produces intra-lamellar crosslinking which inhibits affine deformation by the suppression of slip, but deformation by (110) twinning persists because it involves only very small movements of the molecular chains. A new deformation mechanism was found in irradiated crystals, namely the nucleation and growth of micro-cracks by shear on the (200) and (020) planes of the lamellar crystal. This phenomenon is tentatively attributed to dislocation pile up at crosslinking sites.