The isolation and characterization of a denitrifying bacterium that is both moderately halophilic and alkaliphilic is described. The organism was isolated for use in the development of a bioprocess that could potentially reduce the costs of ion exchange resin regenerant disposal. The process of ion exchange, after resin regeneration, produces a briny, alkaline waste that is difficult and expensive to dispose. The biological removal of nitrate and subsequent reuse of these brines can potentially provide a cost-saving alternative to disposing of this waste product. To achieve our objective, a moderately halophilic, alkaliphilic bacterium was isolated from sediment samples taken from the salt plain of Alkali Lake in Washington State (USA). The haloalkaliphilic bacterium, designated strain 4A, is motile with rod-shaped cells that are 3 to 5 microm long and 1 microm wide. Electron acceptors used include oxygen, nitrate, and nitrite. In addition, it has similar specific nitrate reduction rates and biomass yields as non-halophilic denitrifying bacteria. It is capable of using a variety of electron donors. This organism can grow at NaCl concentrations ranging from 0.2 to 4.5 M with optimum growth occurring at 1.5 M and pH values ranging from 6 to 12 with 9.5 being the optimum pH. The temperature range for growth of strain 4A is 4-50 degrees C with optimal growth occurring at 30 degrees C. The G + C content is 66 mol%. Phylogenetic analyses based upon 16S rDNA gene sequence placed isolate 4A in the genus Halomonas. In addition, DNA-DNA hybridization experiments clearly indicate that it is a unique species. Phenotypic and phylogenetic studies indicate that isolate 4A represents a new species. We propose the name Halomonas campisalis for this species and strain 4A (ATCC 700597) as the type strain. Due to its denitrification ability, broad carbon utilization range and its high salinity and pH tolerance this organism, and similar ones, hold promise for the treatment of saline, alkaline waste.