This work compared the elastomeric properties of two low-crystallinity ethylene-octene copolymers. One was a block copolymer with lamellar crystals and the other was a random copolymer with fringed micellar crystals. The comparison of the stress-strain behavior at 23 degrees C revealed that the initial elastic modulus and the yield stress depended only on the crystallinity of the copolymer. When the temperature was raised above 23 degrees C, melting of the fringed micellar crystals of the random copolymer caused a rapid decrease in the modulus. Some decrease in the modulus of the block copolymer over the same temperature range was attributed to the crystalline alpha-relaxation. Both polymers exhibited strain-hardening, ultimate fracture at high strains, and high recovery after fracture. However, in the block copolymer, the onset of strain-hardening and the ultimate fracture occurred at higher strains. The block copolymer also showed higher recovery from high strains. The initial stretching resulted in a permanent change in the stress-strain Curve. It was suggested that following the onset of crystal slippage at the yield, the crystals underwent permanent structural changes through the course of the strain-hardening region. The transformation of the fringed micellar crystals occurred at lower strains than the transformation of the lamellar crystals. The extent of the structural transformation was described by the crosslink density and the strain-hardening coefficient extracted from elasticity theory. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3236-3244, 2009