A bacterial strain designated CMJ-9(T) was isolated from a freshwater shrimp culture pond in Taiwan and characterized using a polyphasic taxonomic approach. Cells of strain CMJ-9(T) were strictly aerobic, Gram-negative, motile by a single polar flagellum, poly-�]-hydroxybutyrate-containing and formed light-yellow colonies. Growth occurred at 10-37 �XC (optimum, 20-30 �XC), with 0-0.8 % NaCl (optimum, 0-0.1 %) and at pH 6.0-9.0 (optimum, pH 6.0-7.0). Phylogenetic analyses based on 16S rRNA gene sequences showed that strain CMJ-9(T) belonged to the genus Undibacterium, and its closest neighbour was Undibacterium seohonense SHS5-24(T), with 96.7 % sequence similarity. The predominant cellular fatty acids were summed feature 3 (C16 : 1�s7c and/or C16 : 1�s6c) and C16 : 0. The major cellular hydroxy fatty acid was C10 : 0 3-OH. The polar lipid profile consisted of the predominant lipids phosphatidylethanolamine, phosphatidylglycerol and diphosphatidylglycerol. The polyamine profile was composed of the major compound putrescine and moderate amounts of 2-hydroxyputrescine. The major respiratory quinone was Q-8 and the DNA G+C content was 47.7 mol%. On the basis of the phylogenetic and phenotypic data, strain CMJ-9(T) should be classified within a novel species, for which the name Undibacterium macrobrachii sp. nov. is proposed. The type strain is CMJ-9(T) (= BCRC 80406(T) = LMG 26891(T) = KCTC 23916(T)).