A Gram-negative, aerobic, halophilic, neutrophilic, rod-shaped, non-pigmented, polar-flagellated bacterium, UST010306-043(T), was isolated from a pearl-oyster culture pond in Sanya, Hainan Province, China in January 2001. This marine bacterium had an optimum temperature for growth of between 33 and 37 degrees C. On the basis of 16S rRNA gene sequence analysis, the strain was closely related to Marinomonas aquimarina and Marinomonas communis, with 97.5-97.7 and 97.1 % sequence similarity, respectively. Levels of DNA-DNA relatedness to the type strains of these species were well below 70 %. Analyses of phylogenetic, phenotypic and chemotaxomonic characteristics showed that strain UST010306-043(T) was distinct from currently established Marinomonas species. A novel species with the name Marinomonas ostreistagni sp. nov. is proposed to accommodate this bacterium, with strain UST010306-043(T) (=JCM 13672(T)=NRRL B-41433(T)) as the type strain.