The stress-strain response of low-crystallinity ethylene-octene (EO) and ethylene-styrene (ES) copolymers with 7-20 mol % comonomer was compared over a temperature range that spanned the glass-transition and crystal melting regions. Above the onset temperature of the glass transition, the copolymers exhibited elastomeric behavior with low initial modulus, uniform deformation to high strains, and high recovery after the stress was released. In the glass-transition range, an initial low-stress elastomeric response was followed by a distinct "bump" in the stress-strain curve. On the basis of the temperature and rate dependence of the stress-strain curve, local strain-rate measurements, local temperature changes, and recovery characteristics, the "bump" was identified as high strain yielding. Hence, the stress-strain curve sequentially exhibited the features of elastomeric and plastic deformation. Following high strain yielding, strain hardening dramatically increased the fracture strength. This behavior was defined as elastomeric-plastic. Elastomeric-plastic behavior in the broad glass-transition range constituted a gradual transition from elastomeric behavior at higher temperatures to low-temperature plastic behavior with high modulus and macroscopic necking. Because of the lower glass-transition temperature of EO, -40 degreesC as compared with - 10 degreesC for ES, the onset of elastomeric-plastic behavior occurred at a significantly lower temperature. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions that provides the structural basis for elastomeric behavior of low-crystallinity ethylene copolymers was extended to elastomeric-plastic behavior by considering a network with a fraction of rigid, glassy chains.