Summary. The acetylene reduction technique was used for a 3‐year period to monitor potential nitrogen fixation by aerobic heterotrophic bacteria in the sea 2 miles offshore in Cardigan Bay. Samples from depths down to 15 m were membrane‐filtered and the residues incubated aerobically or anaerobically in acetylene‐containing gas mixtures in sealed Millipore Field Monitors. Pure cultures of aerobic heterotrophs isolated from spread‐plates or monitor membranes supplied with ‘nitrogen‐free’media, were tested for ability to reduce acetylene. Glucose was the best of 14 substrates tested to support both growth and acetylene reduction by marine bacteria. The results suggested the presence of a few aerobic or facultatively anaerobic nitrogen fixers among much more numerous and efficient ‘fixed‐nitrogen‐scavengers’. The acetylene‐reducing capacity of pure cultures was inexplicably variable, even under closely‐standardized conditions: this is discussed. The more consistent acetylene reducers included various‐sized rods (Gram positive and negative), coccobacilli and yeasts, which latter may have had non‐culturable bacteria associated with them. No recognizable Azotobacter sp. was isolated, despite the strongly‐selective conditions imposed. Similar results were obtained for seawater samples from the Irish Sea and an Iceland fjord.