<jats:p>The alga Cystophora moniliformis contains two new bisnorsesquiterpenes, (5E,9E)-5,6-epoxy-6,10,14-trimethylpentadeca-9,13-dien-2-one (3) and its cyclized derivative (4), and the (2E) and (2Z) isomers of geranylgeranal epoxide, (5) and (6), respectively. The structures of the new compounds were deduced by spectroscopic means. Geranylacetone (1) was the major metabolite present in the alga.