1. The dependence of the growth rate of a fatigue crack on the stress intensity coefficient at the tip of the crack is described by an exponential function of the da/dN=CΔKn type for all zones of a welded joint. For a given applied stress and realizable values of ΔK the index n in this function has a constant value, differing for each particular zone. 2. The instantaneous and average crack velocities reach their maximum values in the heat-affected zone and their lowest values in the seam metal. The crack growth rate in the parent metal is close to that in the heat-affected zone. 3. The fatigue life of a weld subjected to cyclic (fatigue) loading may to a first approximation be estimated by the n and C values of the parent metal.