A study of the mechanical behavior of isotactic 1,2-poly(E-3-methyl-1,3-pentadiene) (iP3MPD) and of the structural and morphological transformations that take place upon stretching is presented. Amorphous samples do not show viscous flow but show high ductility, strain hardening, and elastomeric behavior. An unusual double yielding appears in the stress- strain curves that may be correlated to occurrence of events typical of yielding at low deformation followed by stress-induced crystallization of a mesomorphic form at higher deformation (200-300%). The yielding stress and an endothermic hysteresis effect at the glass transition simultaneously increase upon aging of the amorphous glass due to a local densification or formation of local order in the amorphous glass upon physical aging. The elasticity is accompanied by the crystallization of the mesophase during deformation and its melting upon relaxation. During successive cycles of tensile deformation and elastic recovery the yielding peak is still observed during every stretching step. The elasticity of iP3MPD is an unusual case of a polymer that shows rubbery elastomeric properties below the glass-transition temperature.