Analysis of injection moulded parts of polystyrene reinforced with the spherical glass particles of various sizes reveals the existence of a number of interesting microstructural features. These include the segregation of particles towards the centre-plane of the cavity, their accumulation close to the free surface and the subsequent occurrence of short-shots, the formation of particle-rich layers adjacent to the perimeter of the moulding and the formation of monolayers of particles attached to the cavity surfaces. Some of these features are consistent with and can be explained by the flow patterns associated with the mould-filling process, namely a Hele-Shaw flow on the plane of the cavity and a fountain flow near the advancing free surface. A mechanism is proposed, according to which the segregation of particles at the cavity mid-plane and their accumulation towards the free surface is the result of the combined action of convection, fountain flow and lateral particle migration. The configuration of the gate is shown to have a marked effect on the observed microstructures in edge-gated mouldings, depending on the ratio of the gate size over the particle size, as is clearly illustrated through X-ray radiography in polydisperse systems. As a result of particle accumulation at the free surface, the formation of particle-rich weld lines in mouldings with inserts is also observed through X-ray radiography. The observed microstructures are shown to be strongly influenced by the size of the particle.