The report presents the results of a study of the fatigue behavior of 5000 series aluminum alloy weldments submerged in seawater. Tests were conducted on plain plate@ transverse butt-welded and longitudinal butt-welded specimens of 5086-H116@5456-H116@ and 5456-H117 aluminum alloys. The specimens were full thickness plates (3/4 in. and 1 in. thick) axially fatigued under a zero-to-tension stress cycle. Sixty fatigue tests were conducted including thirty-nine tests on specimens in marine environment with the remainder in air environment. Results are presented in both tabular and graphical form. S-N diagrams show the significant reduction in fatigue life for both plain plate and weldments at all stress levels as a result of submergence in seawater. Results are further differentiated on the basis of alloy type and weld orientation. Supplemental tests were conducted including fatigue tests of six plain plate specimens of ABS Class C steel. Studies also included the measurement of weld angles at the point of crack initiation in welded specimens@ finite element analysis of a typical butt-welded shape@ distribution of residual stresses and weld quality evaluation.