Five brightly red-pigmented, motile, rod-shaped, extremely halophilic bacteria were isolated from saltern crystallizer ponds in Alicante (two strains) and Mallorca (three strains), Spain. They grew optimally at salt concentrations between 20 and 30% and did not grow below 15% salts. Thus, these isolates are among the most halophilic organisms known within the domain Bacteria. The temperature optimum was 37-47 degrees C. A single, yet to be identified pigment was present, with an absorption maximum at 482 nm and a shoulder at 506-510 nm. The G+C content of the DNA was 66.3-67.7 mol% and, together, they formed a homogeneous genomic group with DNA-DNA similarities above 70%. The 16S rRNA gene sequences were almost identical to sequences recovered earlier from the saltern biomass by amplification of bacterial small-subunit rRNA genes from DNA extracted from the environment. This phylotype, earlier described as 'Candidatus Salinibacter', was shown by fluorescence in situ hybridization to contribute between 5 and 25% of the prokaryote community of the saltern crystallizers. We have therefore succeeded in isolating a bacterium from the natural environment that, although being a major component of the community, was previously known by its phylotype only. Isolation of the organism now allows formal description of a novel genus and species, for which we propose the name Salinibacter ruber gen. nov., sp. nov. The type strain is strain M31T (= DSM 13855T = CECT 5946T).