Examination of the classic Oligocene coral reefs of the Vicentin (Southern Alps, Northern Italy), reveals a low specific diversity community dominated by Actinacis rollei Reuss. In the lower part of the Oligocene succession, above the Eocene-Oligocene boundary, A. rollei can be sporadically associated with few other species. Increasing species diversity occurs at the end of the Rupelian, where the succession is terminated. As a dramatic lowering of sea-water temperature is documented at the Eocene-Oligocene boundary, we interpret the Actinacis "explosion" in the Early Oligocene mainly as a hypothermic effect. The increasing coral species diversity during the Rupelian may then be a response to ameliorating sea-water temperature.