Osirisynes A-F(1-6), highly oxygenated C 4 7 polyacetylenes have been isolated from the sponge Haliclona osiris collected from Guam. These compounds possess a diacetylenic carbinol and an α-acetylenic carboxylic acid as common structural features. The structures of osirisynes have been determined by combined spectroscopic methods. These compounds exhibited moderate cytotoxicity and inhibitory activities against Na + /K + -ATPase and reverse transcriptase (RT).