This paper reports experimental observations on the way polypropylene (PP) and high-density polyethylene (HDPE) can crystallise during flow. Both a deep and a shallow slit geometry were chosen for the rheo-optical study. Preliminary linear viscoelastic rheological tests enabled the temperature window for quiescent crystallisation to be established. Flow-induced crystallisation (FIC) studies were performed in a temperature regime above the normal quiescent crystallisation conditions. In the case of HDPE, FIC occurred during flow at the sidewalls of the slit and in localised regions downstream and the processing pressure increased during the piston movement. In the case of PP, flow-induced crystallisation was generally observed after flow cessation, and the processing pressure did not change during flow. For PP, FIC also occurred preferentially at the walls in the form of elongated crystallites, but the fibres gradually emerged after flow cessation. The difference in the FIC behaviour was attributed to differences in both the rheology and the crystal growth kinetics of the two materials at the particular super-cooling used.