A halo-alkaliphilic, Gram-positive, non-motile bacterium, designated strain 4AGT, was isolated from a mineral pool located in Malvizza, Campania, southern Italy. On the basis of 16S rRNA gene sequence analysis, strain 4AGT was shown to belong to the genus Alkalibacillus within the phylum Firmicutes; its phylogenetic distance from recognized Alkalibacillus species was <95.0 %. Chemotaxonomic data (MK-7 as the major menaquinone; directly cross-linked meso-diaminopimelic acid in the cell wall; phosphatidylglycerol and diphosphatidylglycerol as major polar lipids; iso-C(15 : 0), anteiso-C(15 : 0) and iso-C(17 : 0) as major fatty acids; and glycine betaine and glutamate as major compatible solutes) supported the affiliation of the strain to the genus Alkalibacillus. The results of DNA-DNA hybridization and physiological and biochemical tests allowed genotypic and phenotypic differentiation of strain 4AGT from the two recognized Alkalibacillus species. Strain 4AGT therefore represents a novel species, for which the name Alkalibacillus filiformis sp. nov. is proposed. The type strain is 4AGT (=DSM 15448T=ATCC BAA-956T).