The terrestrial sedimentary record contains ancient microfossils (as old as 3.5 Ga) morphologically similar to present day cyanobacteria. Carbon isotopic analyses of the organic matter present within Precambrian sediments suggest photosynthetic activity amongst these organisms. Herein are reported nitrogen isotope analyses of organic matter from Precambrian cherts : the sediments having the best potential for the preservation of pristine isotopic compositions. It has been suggested that inorganic ammonium might have been a major nitrogen source for primitive organisms in Archaean oceans. The results are consistent with such a model, requiring a different nitrogen biological cycle compared to the current one, and provide new data for evolution of nitrogen metabolism.