Middle Proterozoic organic-rich sediments from the McArthur Basin in northern Australia contain abundant hydrocarbons which are derived from syngenetic kerogen and hence, are representative of the organic remains of microorganisms living at that time. The major classes of hydrocarbons identified were n-alkanes, monomethyl branched alkanes, cyclohexyl alkanes and acyclic isoprenoids. There were also low abundances of pentacyclic triterpanes comprising hopanes and methyl hopanes. Low concentrations of steranes were present in most samples, but like the triterpanes, they were only easily detected in the least thermally altered sediments, and hence were of limited use in detailed assessments of thermal maturity. The presence of steranes in sediments of this age is strong evidence for the existence of eukaryotic organisms as far back as 1690 Ma, although the relatively high abundances of branched alkanes indicates that most of this primitive organic matter was probably derived from prokaryotes.