For the first time data on nitrogen isotopes from the Ordovician-Silurian sedimentary rocks of the Baltic Basin are reported. Supplementary samples come from several regions worldwide. The data reveal the existence of different primary bioproductivity pathways in the Ordovician-Silurian. During the formation of black shale surface waters were oxygen-poor and maintained N-2-fixing primary production indicated by delta N-15 -0.3 parts per thousand on average. The average delta N-15 of kukersite oil shale is +7.4 parts per thousand. The positive delta N-15 values are in accordance with the formation of kukersite in oxic waters, showing that Gloeocapsomorpha prisca was a nitrate-using not N-2-fixing cyanobacterium-like organism. The black shale samples from the deep shelf suggest that seawater, including the photic zone, often suffered from deficiency of oxygen.