This study presents the excavations of 1979-80 at the so-called harbour temple on Insula 37 of the Colonia Ulpia Traiana. The excavations were aimed at augmenting knowledge of the temple in the light of the proposed archaeological/museumpark in Xanten. The sanctuary, a Mediterranean-style temple, was erected in the second quarter of the second century AD, shortly after the foundation of the colony. The deity worshAed here is still unknown. It was pulled down at the end of the third or beginning of the fourth century when its building materials were reused to build the late antique fortress in the central area of the colony. The work includes a catalogue of finds and five pull-out plans.