A new C25 highly branched alkadiene (5) has been isolated from the diatomaceous benthic microbial communities of Hamelin Pool, Western Australia. The structure (5) was determined by detailed n.m.r. and mass spectral analysis, as well as by chemical degradation. Carbon isotopic measurements on this and related isoprenoids were consistent with an origin from diatoms. The role of (5) and related metabolites as biological marker compounds is discussed.