<jats:p>A new botryococcene (5) incorporating an unprecedented ketone functionality has been identified in the lipid extract of an Australian collection of the green alga Botryococcus braunii Kutzing . The structure was established by detailed n.m.r. and mass spectroscopic analysis. A saturated hydrocarbon (8) prepared from this ketone by catalytic hydrogenation and Wolff-Kishner reduction has a different carbon skeleton to that of (9) prepared by hydrogenation of the co-occurring C33 botryococcene.