The mechanical properties of an aluminum silicon alloy reinforced with ceramic fibers has been investigated as part of a much larger program to develop metal matrix composite Pistons for diesel engine applications. Tensile and fatigue tests were carried out over a range of temperatures typical of those experienced during engine operation. The influence on the properties of non-fibrous extrinsic particles, which originate from the fiber manufacturing process, are considered in detail.These data show that the tensile and fatigue characteristics are much improved over those of unreinforced materials at temperatures in the range of the maximum engine operating temperature. The presence of extrinsic defects has little effect on the tensile properties but causes a reduction in the fatigue life, which may be greater than one order of magnitude at a given load. From these data it is clear that tensile properties alone cannot be used to select such materials for use in fatigue critical applications, such as that described here, and that the fatigue characteristics must be carefully considered. The influence of these properties on the thermal fatigue characteristics of diesel pistons is then discussed.