Nitrogen fixation (CH reduction) by blue-green algae occurring on the juvenile lava field of Heimaey, Iceland was examined both in the laboratory (potential at 20ยฐ C and 39ยฐ C) and in the field, three and a half years after the volcanic eruption.Already at this early stage of colonization representatives of unicellular and filamentous heterocystous and non-heterocystous blue-green algae were commonly observed. The predominating algae were Nostoc sp. (20ยฐ C) and Schizothrix sp. - Microcoleus chthonoplastes, (39ยฐ C), the former often in association with the protonemata-rhizoids of moss plants.The potential for nitrogen fixation was recorded at an average rate of 109.2 (20ยฐ C) and 138.1 (39ยฐ C) ng N g h in soil collected from localities randomly distributed over the lava field.Tests for nitrogen fixation performed in situ revealed significant fixation activities in all the eleven localities subject to examination. The activities ranged from 2.8 to 63.4 (mean 21.5) ng N g h and 1.9 to 17.7 (mean 7.9) ng N cm h.All the nitrogen fixation data noted imply that blue-green algae contribute a substantial part of the nitrogen input to the lava. Further, it was found that material incubated under micro-aerophilic conditions exhibited considerably enhanced nitrogenase activity.The role of nitrogen-fixing blue-green algae in general and Nostoc muscorum in particular in being suitable as pioneering organisms preparing the bare lava for ingress of other plants is also discussed.