Thin films of polystyrene (PS) are bonded to copper grids and crosslinked with electron irradiation. When the films are strained in tension regions of local plastic deformation, either crazes or plane stress deformation zones (DZs), nucleate and grow from dust particles. The nature of the local deformation, as well as the local extension ratio lambda, is determined by transmission electron microscopy. The behavior of the PS glass is consistent with its being a network of molecular strands of total density v = v(E) + v(X), where v(E) is the entangled strand density inferred from melt elasticity measurements of uncrosslinked PS and v(X) is the density of crosslinked strands determined from the ratio of the applied electron dose to the electron dose for gelation. When v is less than 4 x 10(25) m(-3) (<1.3v(E)), only crazes are observed whose microstructure is similar to those in uncrosslinked PS. As v increases from 4 x 10(25) to 8 x 10(25) m(-3) (from 1.3 v(E) to 2.5v(E)) shear deformation begins to compete with crazing. As v increases above 8 x 10(25) m(-3), only shear DZs are observed, the strain in which becomes progressively more diffuse as v increases. The X in the crazes and DZs correlate well with X,,,, the maximum extension ratio of a strand in a network of density v computed using the Porod-Kratky model. For crazes In(lambda) similar or equal to 0.9 In(lambda(max)) and for DZs In(lambda) similar or equal to 0.55 In(lambda(max)). The strain at which crack nucleation is first observed increases as v increases from <5% in uncrosslinked PS with v = 3.3 x 10(25) m(-3) to >20% in PS with v = 33 x 10(25) m(-3) (v = 10v(E)); crosslinking to still higher crosslink densities, e.g., v = 14v(E), results in cracks which propagate in a catastrophic manner at low applied strains. An optimum v thus exists, one not too high to suppress local shear ductility but high enough to suppress crazes which can act as crack nucleation sites. These results are compared with previous results on a variety of linear homopolymers, copolymers, and polymer blends that are characterized by a wide range of v (v = v(E)) The transitions from crazing to crazing plus shear and from crazing plus shear to shear only take place at almost identical values of v. In addition the correlation between lambda in the crazes and DZs and lambda(max) for a single network strand is the same for both classes of polymers. This agreement implies that chain scission is the major mechanism by which strands in the entanglement network are removed in forming fibril surfaces. Craze suppression, by either increasing v in the crosslinked polymer or vs in the uncrosslinked ones, is due to the extra energy required to break more main-chain bonds to form these surfaces.