ABSTRACT: In temperate Atlantic waters (18.8 to 20.1°C), biological nitrogen fixation has been demonstrated by 2 independent measurements: ¹⁵N-N₂ incorporation and nifH identification in the DNA and expressed messenger RNA (mRNA). At 2 stations in the western English Channel, bulk waters were incubated with ¹⁵N-N₂. At the high levels of particulate nitrogen (≤11.5 µmol N l^–1), absolute fixation rates of 18.9 ± 0.01 and 20.0 nmol N l^–1d^–1 were determined. While a caveat must accompany the magnitude of the rates presented due to the limited number of data, the presence and activity of diazotrophic organisms in these waters is of ecological significance and may affect current attitudes to nitrogen and carbon budgets. In particular, our estimate of the rate of N fixation (0.35 mmol N m^–2 d^–1) is comparable to that of denitrification rates in UK shelf seas. Molecular analysis identified a diversity of expressed nifH genes, and 21 different prokaryotic nifH transcripts were identified.