Traffic bridges are subjected to variable loads and should therefore be checked on fatigue. Especially low weight materials, such as aluminium, are sensitive to fatigue, because the variable loads is s substantial part of the total load. This paper discusses the fatigue behaviour of an extruded aluminium deck. to determine the fatigue lifetime, the well-known procedure based on S-N curves was applied. In tests, the S-N curve was determined of a welded detail applied often in aluminium bridge decks. In order to check the procedure based on S-N curves, fatigue tests were carried out on a specimen with geometry and loading conditions that are realistic for bridge decks.