Despite a number of previous investigations, the question of what criterion to use to predict ultimate failure in fiber composite laminates is still open to question, and particulary so when the overall loading involves biaxial stresses. We have addressed this question by developing a progressive failure model, and comparing the model to experiments on biaxial loading of tubular specimens of quasi-isotropic AS4/3501-6 carbon/epoxy laminates. The progressive failure model uses a modified Tsai-Wu criterion for initial ply cracking, a rule for softening the ply stiffness coefficients with continued straining, and an ultimate failure criterion. It was found that a maximum fiber strain criterion agreed very well with the experiments.