Fatigue crack propagation has been studied in an Al-2.6Mg alloy between room temperature and 300 C. It is shown that the fatigue crack propagation rate is controlled by volume diffusion. The activation barrier for volume diffusion (Q⁰) is lowered by the applied stress intensity range ΔK so that Q(ΔK) = Q⁰ - C¹ log ΔK. The driving force for the process is inversely proportional to the square of the stress intensity range: A(ΔK) = C² ΔK^-2. Fatigue crack propagation rate as a function of temperature and stress intensity range is given by daDN=C2ΔK-2exp⁡-[Q0-C1log⁡ΔKRT] A special case of this equation is the well-known Paris equation.