The aliphatic hydrocarbon constituents of four blue-green algae, Nostoc muscorum, Anacystis nidulans, Phormidium Iuridium, and Chlorogloea fritschii, have been investigated. The findings indicate that the n-C₁₇ hydrocarbon is the predominant component in the hydrocarbon mixtures. Methyl branchedalkanes seemed to be of particular significance because the methyl branches are not iso- and anteiso-structures. They appear to occur uniquely in blue-green algae and are absent from photosynthetic and non-photosynthetic bacteria. These hydrocarbons have not been found in the green algae, which are more advanced than blue-green algae but are still relatively primitive. Mass spectrometry indicates that the methyl branched alkane from Chlorogloea fritschii may be 4-methylheptadecane. The Nostoc branched C₁₈ compound was studied by i.r., n.m.r., and mass spectrometry. Synthesis of 7,9-dimethylhexadecane and of 7- and 8-methylheptadecanes proved that the branched hydrocarbon consisted of a mixture (1:1) of 7- and 8-methylheptadecanes, which could not be separated by capillary g.l.c.