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Anaerobic nitrogen fixation has been studied in sediments from two Scottish sea-lochs, Loch Eil and Loch Etive. The problem was approached in three ways: 1. Estimation of nitrogen fixation (acetylene reduction) rates. 2. A characterisation of the bacteria involved. 3. An assessment of some chemical and physical factors likely to influence nitrogen fixation. Aerobic nitrogen fixation was not detected. Anaerobic nitrogen fixation rates in the range 0.06-4.10 n-moles ethylene produced. g dry weight-1 hr-1 were measured. In these sea-lochs nitrogen fixation in the sediments appears to be due principally to sulphate-reducing bacteria of the genus Desulfovibrio , which are associated with sulphide deposits and negative Eh. Clostridium spp. are also present in the sediments but do not appear to be active.