We investigated the spatial and temporal variation in denitrification rates (isotope-­pairing technique, IPT) and in the denitrifier community (examination of gene nirK by denaturing-gradient gel electrophoresis [DGGE] of microbial DNA) in the sediments of a boreal, clear-water, eutrophic lake using samples collected from shallow littoral, deep littoral and shallow profundal ­sediments during early summer, mid-summer, autumn and winter. The measured denitrification rates (44 to 613 µmol N m−² d−¹) are among the lowest ever reported from lacustrine sediments. Denitri­fi­cation rates varied both spatially and temporally, being highest in the profundal zone during mid-­summer and in the littoral zones during winter. Correlation analyses indicated that these variations were due to variations in the concentrations of nitrate and oxygen in the water overlying the sediment. The structure of the denitrifier community was temporally extremely stable and differed only slightly between the sites. Distance-based linear model (DISTLM) analysis indicated that the observed variation was probably due mainly to variations in the content of organic matter, and in the porosity, of the sediment. The structure of the denitrifier community and the denitrifying activities were uncoupled.