Aerospace fasteners originally designed for metallic structures have been adopted for composite structures which exhibit z-axis stiffness that is an order of magnitude lower than most of aerospace structural metals. This paper presents a computational investigation of the influences of z-axis stiffness on the stress distribution on metallic fasteners in composite joints. The influence of z-axis stiffness on the bearing stresses of countersink fasteners and the consequent head stresses are discussed. Results from common aerospace structural metals are used in comparison with typical CFRP configurations found in aircraft structures.