The denitrification capability of Cluster I Defluviicoccus vanus-related glycogen - accumulating organisms (DvGAOs) is investigated. A sequencing batch reactor (SBR) fed with acetate as the sole carbon source was operated under alternating anaerobic-aerobic conditions to enrich Cluster I DvGAOs. Fluorescence in situ hybridization (FISH) showed that more than 85% of the bacterial population present in the reactor bound to the probes previously designed for Cluster I DvGAOs. A series of batch tests were performed to evaluate the capability of the community to reduce nitrate and nitrite. The tests were carried out both before and after the adaptation of the culture to anoxic conditions, and with both the intracellularly stored carbon and acetate as the electron donors. It was found that Cluster I DvGAOs were able to reduce nitrate but most likely unable to reduce nitrite. When un-adapted Cluster 1 DvGAOs were exposed to nitrate for the first time, a lag phase of approximately 4 h occurred, which was likely required for the synthesis of the necessary enzymes.