A numerically important member of marine enrichment cultures prepared with lignin-rich, pulp mill effluent was isolated. This bacterium was gram negative and rod shaped, did not form spores, and was strictly aerobic. The surfaces of its cells were covered by blebs or vesicles and polysaccharide fibrils. Each cell also had a holdfast structure at one pole. The cells formed rosettes and aggregates. During growth in the presence of lignocellulose or cellulose particles, cells attached to the surfaces of the particles. The bacterium utilized a variety of monosaccharides, disaccharides, amino acids, and volatile fatty acids for growth. It hydrolyzed cellulose, and synthetic lignin preparations were partially solubilized and mineralized. As determined by 16S rRNA analysis, the isolate was a member of the alpha subclass of the phylum Proteobacteria and was related to the genus Roseobacter. A signature secondary structure of the 16S rRNA is proposed. The guanine-plus-cytosine content of the genomic DNA was 65.0 mol%. On the basis of the results of 16S rRNA sequence and phenotypic characterizations, the isolate was sufficiently different to consider it a member of a new genus. Thus, a novel genus and species, Sagittula stellata, are proposed; the type strain is E-37 (= ATCC 700073).