||We designed and tested a set of specific primers for specific PCR amplification of the biotin carboxylase subunit gene (accC) of the Acetyl CoA carboxylase (ACCase) enzyme. The primer set yielded a PCR product of c. 460 bp that was suitable for denaturing gradient gel electrophoresis (DGGE) fingerprinting followed by direct sequencing of excised DGGE bands and sequence analysis. Optimization of PCR conditions for selective amplification was carried out with pure cultures of different bacteria and archaea, and laboratory enrichments. Next, fingerprinting comparisons were done in several aerobic and anaerobic freshwater planktonic samples. The DGGE fingerprints showed between 2 and 19 bands in the different samples, and the primer set provided specific amplification in both pure cultures and natural samples. Most of the samples had sequences grouped with bacterial accC, hypothetically related to the anaplerotic fixation of inorganic carbon. Some other samples, however, yielded accC gene sequences that clustered with Crenarchaeota and were related to the 3-hydroxypropionate/4-hydroxybutyrate cycle of autotrophic crenarchaeota. Such samples came from oligotrophic high mountain lakes and the hypolimnia of a sulfide-rich lake, where crenarchaeotal populations had been previously reported by 16S rRNA surveys. This study provided a fast tool to look for presence of accC genes in natural environments as potential marker for studies of carbon dioxide assimilation in the dark. After further refinement for better specificity against archaea, the new and novel primers could be very helpful to establish a target for crenarchaeota with implications for our understanding of archaeal carbon biogeochemistry.