Biomarker hydrocarbons in oils and sediments have considerable potential for assessing the nature of biological processes in the past, but only a few can be uniquely related to particular organisms. Reef-hosted oils in the Silurian of the Michigan Basin, Canada and the Devonian of Western Canada have high abundances of 1-alkyl-2,3,6-trimethylbenzenes. Their precise structures suggest that they are diagenetic products of aromatic carotenoids of the green sulphur bacteria. Individual components are enriched in 13C by 7-8‰ relative to the saturated hydrocarbons of the same oil. An isotopic anomaly of this magnitude and direction correlates with a component of photosynthetic carbon assimilation by the reductive tricarboxylic acid cycle. These results, together with data on palaeoenvironments, provide compelling evidence for the existence, in ancient restricted seas, of microbial communities containing Chlorobiaceae, organisms with distinctive biochemistry which leave morphologically identifiable remains.