A heterotrophic, antibiotic-producing bacterium, strain T5(T), was isolated from the German Wadden Sea, located in the southern region of the North Sea. Sequence analysis of the 16S rRNA gene of this strain demonstrated close affiliation with Roseobacter gallaeciensis BS107(T) (99 % similarity), but the results of genotypic (DNA-DNA hybridization and DNA G + C content) and phenotypic characterization revealed that strain T5(T) represents a novel species. The novel organism is strictly aerobic, Gram-negative, rod-shaped, motile and forms brown-pigmented colonies. Strain T5(T) produces the antibiotic tropodithietic acid throughout the exponential phase which inhibits the growth of bacteria from different taxa, as well as marine algae. Strain T5(T) requires sodium ions and utilizes a wide range of substrates, including oligosaccharides, sugar alcohols, organic acids and amino acids. The DNA G + C content is 55.7 mol%. Comparative 16S rRNA gene sequence analysis revealed that strains T5(T) and Roseobacter gallaeciensis BS107(T) group with Leisingera methylohalidivorans as their closest described relative within the Roseobacter clade (97.9 and 97.6 % sequence similarity, respectively) and with Ruegeria algicola (96.6 and 96.5 % similarity, respectively) of the Alphaproteobacteria. Comparison of strains T5(T) and Roseobacter gallaeciensis BS107(T) with Roseobacter denitrificans and Roseobacter litoralis showed striking differences in 16S rRNA gene sequence similarities, chemical composition, pigmentation, presence of bacteriochlorophyll a and antibiotic production. On the basis of these results, it is proposed that Roseobacter gallaeciensis is reclassified as the type species of a new genus, Phaeobacter, as Phaeobacter gallaeciensis comb. nov. (type strain BS107(T) = CIP 105210(T) = ATCC 700781(T) = NBRC 16654(T) = DSM 17395(T)). Strain T5(T) (=LMG 22475(T) = DSM 16374(T)) is proposed as the type strain of a novel species of this genus, Phaeobacter inhibens sp. nov. At the same time, emended descriptions are provided of the genera Roseobacter, Ruegeria and Leisingera, as well as reclassifying Ruegeria algicola as the type species of a new genus, Marinovum, with the name Marinovum algicola comb. nov.