The Montreal Biodome operates a methanol-fed denitrification system that treats the water in its three million litre marine mesocosm. An unknown bacterium, named strain NL21T, was isolated from this system on TSA and R2A agar. The organism is a Gram-negative, rod-shaped (1x3 microm) facultative aerobe. Optimal growth conditions on R2A agar are 30-35 degrees C, pH 7-7.5 and 1% (w/w) NaCl. Phylogenetic analysis of the 16S rDNA sequence reveals that strain NL21T forms a novel lineage in the family 'Phyllobacteriaceae' within the alpha2 subgroup of the Proteobacteria. The closest related genera are Aminobacter, Pseudaminobacter, Mesorhizobium and Defluvibacter. Major cellular fatty acids are C(18:1)omega7c (75%), C(19:0)omega8c cyclopropane (9.4%) and C(18:0) (4.2%). The DNA G+C content of strain NL21T (57 mol%) differs from those of all other described members of the 'Phyllobacteriaceae' (60-64 mol%). Strain NL21T reduces nitrate to nitrite, but does not reduce nitrite to nitrogen gas. Only a few sugars and amino acids can serve as carbon sources. Strain NL21T is able to grow without salt and tolerates up to 5% NaCl. Phylogenetic analysis, as well as physiological and biochemical tests, showed that strain NL21T was different from all other members of the 'Phyllobacteriaceae' with validly published names. Strain NL21T therefore represents a novel genus, for which the name Nitratireductor aquibiodomus gen. nov., sp. nov. is proposed, with the type strain NL21T (=DSM 15645T=ATCC BAA-762T).