The effects of particulate reinforcement on the fatigue behaviour a nd fatigue mechanisms of two 6061 aluminium-based metal-matrix composites (MMCs) in three different heat-treatment conditions were studied in situ with a scanning electron microscope and compared to the unreinforced alloy in the as-received condition. It was observed that the; fatigue properties of the MMCs were influenced by the ceramic particles in two ways : firstly the particles increased the fatigue stress intensity threshold mainly by crack-deflection and crack-closure mechanisms, and secondly, the particles raised the fatigue crack growth rates in the Paris region by providing an easy crack path. The effect of ageing was small on the fatigue stress intensity threshold of MMCs, but for the peak-aged MMCs the fatigue crack growth rates in the Paris region were faster. The mechanism of fatigue crack growth was largely associated with the matrix/particle interface and the linkage with subcracks initiated ahead of the main crack at high applied-stress intensity factors.