A novel β-agrase (EC 3.2.1.81) was purified from an agar-degrading alkalophilic bacterium, Alteromonas sp. E-1 isolated from the soil. This enzyme was obtained from a cell-free extract after sonication and purified 40.9-fold through treatment with streptomycin, ammonium sulfate fractionation and successive chromatography on anion-exchange and gel filtration columns. The molecular weight was estimated to be 82 kDa by SDS-polyacrylamide gel electrophoresis and 180 kDa by Superdex 200 gel filtration. The enzyme was inhibited by Mn 2 + , Cu 2 + , Fe 2 + , Zn 2 + and Hg 2 + , and activated by K + , Na + and EDTA, and its optimum pH and temperature for agarose degradation were 7.5 and 40 o C, respectively. This β-agarase hydrolyzed agarose with rapid reduction of viscosity, and neoagarobiose [O-3,6-anhydro-a-l-galactopyranosyl(1->3)-d-galactose] was detected from the early stage of the reaction. Neoagarobiose as the final product was selectively released from agarose, neoagarohexaose and neoagarotetraose by the reaction with this β-agarase. This observation was different from that of other β-agarases which produced mixtures of neoagarobiose and neoagarotetraose as the final hydrolysis products. The N-terminal amino acid sequence of this β-agarase shows no homology to those of other β-agarases that were so far reported.