Under tensile strain the inhomogeneous surface microstructure of thin polystyrene films gave rise to the formation of ∼300 Å nuclei in which large strains were localized even at the glass transition. At lower temperatures these nuclei coalesced into disk-shaped microneck zones. Fibril formation within these zones and a stable craze structure was possible only with molecular weights higher than 10,000 to 37,500.