Modulus reduction and damage accumulation during fatigue cycling of an injection moulded composite been studied. The material is a long, discontinuous glass fibre reinforced nylon composite having a skin/core structure. A continual decrease in modulus with cycling is observed for composite and single skin layer specimens. During fatigue cycling of composite specimens damage accumulates in both the skin and core layers and both layers contribute to the measured modulus reduction. Damage accumulation in the core, in the form of transverse cracks, is studied in detail. These cracks initiate predominantly at the free edges and grow across the specimen width with cycling. A milling/dye penetrant technique is developed to quantify the cracking and average crack spacings are related to measured modulus reductions at a range of peak stresses. The experimental data is compared with predictions made using a shear-lag model developed for continuous fibre cross-ply laminates relating modulus reduction to average crack spacing. A theoretical model, based on the Cox model, is derived to predict modulus reduction/cycling curves for skin layer specimens. Cracking patterns in the core are predicted using a strain energy release rate approach which is compared with experimental data. Empirical relationships, derived for the modulus reduction of the skin and core layers are combined to predict modulus reduction/cycling curves at a range of specimen widths and stress levels.