The ammonia-producing bacteria B55(T), CA73, SA69 and SA72 were isolated from the waterweeds Ludwigia adscendens (B55(T)) and Eleocharis dulcis (CA73, SA69 and SA72) grown in highly acidic swamps (pH 2-4) in actual acid sulfate soil areas of Vietnam. The isolates were Gram-positive, irregular rod-shaped, non-spore-forming bacteria. On the basis of 16S rRNA gene sequence similarity, strain B55(T) was shown to belong to the genus Curtobacterium of the class Actinobacteria. Chemotaxonomic data (MK-9 as major isoprenoid quinone, d-ornithine as cell-wall diamino acid, acetyl as the acyl type of peptidoglycan) supported the affiliation of all four strains to this genus. Although their 16S rRNA gene sequence similarity was 99 % to species with validly published names within the genus, they formed a group that was distinct in the phylogenetic tree, and DNA-DNA relatedness values to these established species were less than 10 %. The results of physiological and biochemical tests and major fatty acids (cyclohexyl-C(17 : 0), anteiso-C(17 : 0) and cyclohexyl-C(19 : 0)) allowed phenotypic differentiation of these strains from the species of Curtobacterium with validly published names. Therefore, strains B55(T), CA73, SA69 and SA72 represent a novel species, for which the name Curtobacterium ammoniigenes sp. nov. is proposed. The type strain is B55(T) (=NBRC 101786(T)=VTCC D6-11(T)=JCM 14609(T)).