In view of preparing a Flora of the Seaweeds of N. Papua New Guinea, material was collected by scuba-diving along the coast of Madang Province in June-August 1980 and July-August 1986. The Caulerpales are the first group to be studied. Thirteen taxa belonging to the genus Caulerpa are described and illustrated; ecological and biogeographical data are added. The list includes C. brachypus, C. elongata, C. filicoides var. andamanensis, C. lentillifera var. kilneri, C. manorensis, C. microphysa, C. opposita sp. nov., C. racemosa var. clavifera, C. racemosa var. peltata, C. serrulata var. serrulata C. serrulata var. pectinata, C. sertularioides, and C. verticillata.