An extracellular enzyme (RMEBE) possessing ${alpha}-(1{ ightarrow}4)-(1{ ightarrow}6)$-transferring activity was purified to homogeneity from Rhodothermus marin us by combination of ammonium sulfate precipitation, Q-Sepharose ion-exchange, and Superdex-200 gel filtration chromatographies, and preparative native polyacrylamide gel electrophoresis. The purified enzyme had an optimum pH of 6.0 and was highly thermostable with a maximal activity at $80^{circ}C$. Its half-life was determined to be 73.7 and 16.7 min at 80 and $85^{circ}C$, respectively. The enzyme was also halophilic and highly halotolerant up to about 2M NaCl, with a maximal activity at 0.5M. The substrate specificity of RMEBE suggested that it possesses partial characteristics of both glucan branching enzyme and neopullulanase. RMEBE clearly produced branched glucans from amylose, with partial ${alpha}-(1{ ightarrow}4)$-hydrolysis of amylose and starch. At the same time, it hydrolyzed pullulan partly to panose, and exhibited ${alpha}-(1{ ightarrow}4)-(1{ ightarrow}6)$-transferase activity for small maltooligosaccharides, producing disproportionated ${alpha}-(1{ ightarrow}6)$-branched maltooligosaccharides. The enzyme preferred maltopentaose and maltohexaose to smaller maltooligosaccharides for production of longer branched products. Thus, the results suggest that RMEBE might be applied for production of branched oligosaccharides from small maltodextrins at high temperature or even at high salinity.