The results of an experimental study on the fatigue behavior of transverse welded steel butt joints containing slag inclusion discontinuities are presented. Twenty‐four constant amplitude zero‐to‐tension fatigue tests were conducted. The 5 in. (127 mm) wide by 1 in. (25 mm) thick quenched and tempered steel specimens were manually welded using the shielded metal arc process. The size, shape, location, and number of slag inclusions entrapped in the welds were varied to examine the relative importance of these factors on the fatigue behavior. The onset of fatigue crack initiation was estimated by monitoring strain gages on the weld surface near the entrapped slag and by examining records of the ultrasonic signal from the slag inclusions. The test results indicate that: (1) Approximately one‐fourth of the total fatgue life was spent in initiating a fatigue crack, and that nearly 55% of the fatigue life was required for propagation of a crack to the weld surface; (2) a number of fatigue cracks initiated at sm...