BECAUSE their molecular signatures are often prominent in extracts of sediments, bacteria are thought to be important contributors to petroleum source beds(1). It has been shown recently(2,3), however, that abundances of biomarkers do not always reflect relative contributions to sedimentary organic carbon (C-org). The contribution of photosynthetic green sulphur bacteria to sediments can be assessed effectively because the diagenetic products of distinctive carotenoids from these organisms occur widely(4-11) and their biomass is isotopically labelled, being enriched in C-13 (refs 11, 13). We show here that, although sediments and oils from the Western Canada and Williston basins contain prominent biomarkers of photosynthetic bacteria, the absence of C-13 enrichment in the total C-org requires that the bacterial contribution is in fact minimal. Although the importance of bacterial reworking of sedimentary debris cannot be doubted(14), we argue that our findings, when considered in conjunction with those from other settings, suggest that bacterial biomass may commonly represent only a minor component of total C-org in carbonaceous rocks.