By solid-state NMR and X-ray diffraction techniques, monoclinic crystallites are found to exist in ethylene copolymers with high comonomer content and bulky side groups. In the copolymers, the monoclinic form can nucleate and grow under quiescent conditions, at ambient temperature and pressure. The monoclinic:orthorhombic population ratio increases with comonomer content and cooling rate during crystallization. Compared to the orthorhombic form, monoclinic crystallites have a lower average melting point and possibly a smaller thickness. It is proposed that the formations of the monoclinic crystalline structures in ethylene copolymers and in deformed linear polyethylene share a common mechanism: the monoclinic nuclei are favored by the crystalline-amorphous interface, which increases in importance as the nucleus size decreases at increasing comonomer content or as a result of shear or fast cooling.