A Gram-stain-positive, non-motile, chemo-organotrophic, mesophilic, aerobic bacterium, designated A1(T), was isolated from sludge of a wastewater treatment plant. Strain A1(T) showed good ability to degrade ammonia and grew well on media amended with methanol and ammonia. Strain A1(T) grew with 0-11 % (w/v) NaCl, at 20-42 °C, but not <15 or >45 °C and at pH 6-10 (optimum pH 8.0-9.0). The isolate was catalase-positive and oxidase-negative. The DNA G+C content was 70.7 mol%. A comparative analysis of 16S rRNA gene sequences revealed that strain A1(T) formed a distinct phyletic lineage in the genus Brevibacterium and showed high sequence similarity with Brevibacterium casei NCDO 2048(T) (96.9 %), Brevibacterium celere KMM 3637(T) (96.9 %) and Brevibacterium sanguinis CF63(T) (96.4 %). DNA-DNA hybridization revealed <43 % DNA-DNA relatedness between the isolate and its closest phylogenetic relatives. The affiliation of strain A1(T) with the genus Brevibacterium was supported by the chemotaxonomic data: predominant quinone menaquinone MK-7(H2); polar lipid profile containing diphosphatidylglycerol, phosphatidylglycerol and an unidentified glycolipid; characteristic cell-wall diamino acid meso-diaminopimelic acid; whole-cell sugars galactose, xylose and ribose; absence of mycolic acids; and major fatty acids iso-C15 : 0, anteiso-C15 : 0 and anteiso-C17 : 0. The results of physiological and biochemical tests allowed phenotypic differentiation of strain A1(T) from members of the genus Brevibacterium. On the basis of the results in this study, a novel species, Brevibacterium ammoniilyticum sp. nov., is proposed. The type strain is A1(T) ( = KEMC 41-098(T) = JCM 17537(T) = KACC 15558(T)).