Low-temperature (220-290 K) ethylene-ran-propylene copolymer crystallization was followed by DSC and solid-state NMR spectroscopy that can detect the formation of even small and short-living crystalline aggregates (minimum lifetime of 3 ms). NMR at 282 K reveals a sharp and reversible melting-crystallization transition in the copolymer with 74% ethylene units. The trans conformation fraction identifying the aggregates is detected at all temperatures independent of the thermal history, as at each temperature a quick gauche/trans equilibration is realized with no evidence of hysteresis. Transition occurs in both ethylene homosequences and heterosequences, indicating the inclusion of propylene units in the aggregates. The heterosequences do not aggregate in a separate phase, proved by the fast equilibration of the hydrogen magnetization to the same relaxation times as for the homosequences (T-1rho(H-1) value of approximate to4 ms). Both chain mobility and fast carbon spin-lattice relaxation times (T-1(C-13)approximate to1 s) are promoted by the random distribution along the chain of the propylene units that push the vicinal chains apart. The copolymer chain segment behavior in the aggregates is dynamic, similar to that of polyethylene, as arranged in the hexagonal packing obtained at high temperature and pressure and to that of polyethylene mesophases confined to crystalline nanochannels.