A novel Streptomyces, strain MUSC 26T, was isolated from mangrove soil at Tanjung Lumpur, Malaysia. The bacterium was observed to be Gram-positive and to form grayish yellow aerial and substrate mycelium on ISP 7 agar. A polyphasic approach was used to study the taxonomy of strain MUSC 26T, which shows a range of phylogenetic and chemotaxonomic properties consistent with those of the members of the genus Streptomyces. The cell wall peptidoglycan was determined to contain ll-diaminopimelic acid. The predominant menaquinones were identified as MK-9 (H8) and MK-9(H6). The polar lipids detected were identified as diphosphatidylglycerol, phosphatidylinositol, phosphatidylethanolamine, hydroxyphosphatidylethanolamine, phosphatidylmethylethanolamine and hydroxyphosphatidylmethylethanolamine. The predominant cellular fatty acids (>10.0 ) were identified as anteiso-C15:0 (31.4 ), iso-C16:0 (16.3 ), iso-C15:0 (13.9 ) and anteiso-C17:0 (12.6 ). The cell wall sugars were found to be galactose, glucose, mannose, ribose and rhamnose. These results suggest that MUSC 26T should be placed within the genus Streptomyces. Phylogenetic analysis indicated that closely related strains include Streptomyces qinglanensis 172205T (96.5 sequence similarity), S. sodiiphilus YIM 80305T (96.5 ) and S. rimosus subsp. rimosus ATCC 10970T (96.4 ). DNA?DNA relatedness values between MUSC 26T and closely related type strains ranged from 17.0 ? 2.2 to 33.2 ? 5.3 . Comparison of BOX-PCR fingerprints indicated MUSC 26T presents a unique DNA profile. The DNA G+C content was determined to be 74.6 mol . Based on this polyphasic study of MUSC 26T, it is concluded that this strain represents a novel species, for which the name Streptomyces gilvigriseus sp. nov. is proposed. The type strain is MUSC 26T (=DSMZ 42173T = MCCC 1K00504T).