A nested PCR primed by four degenerate oligonucleotides was developed for the specific amplification of sequences from the narG gene encoding the membrane-bound nitrate reductase, This approach was used to amplify fragments of the narG gene from five Pseudomonas species previously shown to be able to express the membrane-bound nitrate reductase and from community DNA extracted from a freshwater sediment. Amino acid sequences encoded by the narG fragments:were compared to one another, and to the corresponding regions of related enzymes. This comparison indicates that the amplification protocols are specific for their intended targets. Sequences amplified from community DNA were tightly clustered, which may indicate a degree of homogeneity in the sediment community. The PCR primers and amplification protocols described will be useful in Future studies of nitrate respiring populations. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.