The internal morphology of isothermally bulk‐crystallized fractions, and of whole polymer, Marlex 50 polyethylene is revealed by fracture experiments. Three distinctly different types of lamellae are then observable if one uses suitable electron microscopic techniques: (1) regular or type I lamellae which are similar in appearance to solution‐grown lamellae, (2) narrow or type II lamellae which have finite widths, and (3) extended‐chain or type III lamellae which have step heights approximately equal to fully extended chain lengths calculated from viscosity‐average molecular weights. The type I and II lamellae appear to consist of folded chains. The step heights of the type I and type II lamellae are shown to increase from about 150 to 350 Å with increasing crystallization temperature. The widths of the type II or narrow lamellae vary directly with temperature from 350 to 800 Å. However, the step heights of the type III lamellae, which range from 150 to 1050 Å, are essentially independent of temperature but do vary with molecular weight. The melting points of these different types of lamellae are shown to be consistent with presently existing theories and the relationship between morphological parameters of these lamellae and reported low‐angle x‐ray periodicities are discussed.