Development of earlywood and latewood fibres was investigated to find out how morphologically different fibres undergo delamination. Fibre fractions rich in earlywood and latewood, were separated from mechanical pulps using a hydrocyclone and refined further in awing defibrator. Changes in fibre structure due to defibration were studied using microscopy techniques that included measurement of fibre stiffness, fibre wall thickness and external fibre surface. Before refining, the latewood fibres were stiff and their external fibre wallswere poorly developed. Refining reduced the stiffness of both fibre types. The stiffness of latewood fibres decreased to around that of unrefined earlywood fibres, andthe external walls of latewood fibres became fibrillated. The wall thickness of both earlywood and latewood fibres was reduced only slightly. Although the tensile and tear indices of sheets made of late wood fibres were improved by refining, the tensile index of flexible latewood fibres was only half of that measured for unrefined earlywood fibres. This indicates that there are fibre properties other than stiffness which must be changed in order to get latewood fibres to bond and conform properly.