Two novel Gram-negative bacteria, designated strains DR41_21(T) and DR41_18(T), were isolated from coastal, surface waters of the north-western Mediterranean Sea. The cells were motile, pleomorphic rods, 2.9 mu m long and 0.9 mu m wide and formed cream colonies on marine agar medium. The G + C content of the genomic DNA was 60 mol%. Phylogenetic analysis of 16S rRNA gene sequences positioned the isolates in the class Alphaproteobacteria within the family Rhodospirillaceae. The 16S rRNA gene sequence similarity of the two strains was 98.8% but DNA-DNA hybridization indicated only 55% relatedness. Strain DR41_21(T) was able to denitrify and possessed nirK and nosZ genes, unlike strain DR41_18(T), which possessed only nirK These isolates represent two novel species of a new genus, Nisaea gen. nov., for which the names Nisaea denitrificans sp. nov. and Nisaea nitritireducens sp. nov. are proposed. The type strain of Nisaea denitrificans is DR41_21(T) (=DSM 18348(T) = CIP 109265(T) = OOB 129(T)) and the type strain of Nisaea nitritireducens is DR41_18(T) (=DSM 19540(T) = CIP 10960(T) = OOB 128(T)).