The failure mechanisms in tension and fatigue of three Al alloys reinforced with ceramic particulates were studied by means of optical and scanning electron microscopy. Damage was concentrated in the reinforcements, which failed in a brittle fashion during deformation, leading to the specimen fracture when a critical fraction of broken particulates was reached in a given section of the specimen. This critical fraction was measured on polished longitudinal sections of broken specimens for each composite, temper, and loading condition, and was mainly dependent on the matrix alloy. It was also found that the reinforcement fracture probability was controlled by the particulate size and aspect ratio : large and elongated particulates oriented in the loading direction were more prone to fail than small, equiaxed ones. Finally, a significant fraction of the reinforcements in the cast materials was broken prior to testing. They were shattered during extrusion rather than fractured, and associated with clusters of particulates formed during solidification.