From the red alga Laurencia okamurai four new C15 acetylenic polyenes, laurencenyne 6, neolaurencenyne 7, trans-laurencenyne 8, and trans-neolaurencenyne 9 have been isolated and their structures were elucidated by chemical and spectral means. Synthesis of these four compounds has been made to confirm their structural and stereochemical assignments. Biogenesis of laurencenyne 6 and trans-laurencenyne