The melt flow of a variety of poly(vinyl chloride) (PVC) samples was studied at shear rates of10–5000 sec−1in a constant‐load capillary rheometer. Pronounced differences were observed in apparent viscosity, post‐extrusion swelling, and extrudate roughness, depending upon sample preparation and previous history. These differences could not be correlated with molecular weight, molecular weight distribution, or other parameters of molecular structure. A possible explanation was suggested by certain correlations between flow behavior and particle structure or state of fusion. Fracture‐surface electron photomicrographs of molded or extruded PVC samples provided clear evidence that resin particles, as formed during polymerization, can maintain their identity and shape during melt flow processes. The melt flow of PVC under some conditions thus must involve the slippage of resin particles past one another, rather than a homogeneous deformation of the melt. Thermal and mechanical history severe enough to obliterate particle identity in fracture‐surface photos also shifts melt flow behavior toward higher apparent viscosity and greater melt elasticity, due to a reduction of particle slippage and a more homogeneous flow process. Particle boundaries may be viewed as discontinuities in an entanglement network, and elimination of boundaries as an extension of the network by molecular diffusion. On this basis, our results demonstrate the important contribution of entanglement distribution to polymer melt rheology.