An extracellular beta-agarase (AgaA34) was purified from a newly isolated marine bacterium, Agarivorans albus YKW-34 from the gut of a turban shell. AgaA34 was purified to homogeneity by ion exchange and gel filtration chromatographies with a recovery of 30% and a fold of ten. AgaA34 was composed of a single polypeptide chain with the molecular mass of 50 kDa. N-terminal amino acid sequencing revealed a sequence of ASLVTSFEEA, which exhibited a high similarity (90%) with those of agarases from glycoside hydrolase family 50. The pH and temperature optima of AgaA34 were pH 8.0 and 40 degrees C, respectively. It was stable over pH 6.0-11.0 and at temperature up to 50 degrees C. Hydrolysis of agarose by AgaA34 produced neoagarobiose (75 mol%) and neoagarotetraose (25 mol%), whose structures were identified by matrix-assisted laser desorption ionization time-of-flight mass spectroscopy and C-13 NMR. AgaA34 cleaved both neoagarohexaose and neoagarotetraose into neoagarobiose. The k(cat)/K-m values for hydrolysis agarose and neoagarotetraose were 4.04x10(3) and 8.1x10(2) s(-1) M-1, respectively. AgaA34 was resistant to denaturing reagents (sodium dodecyl sulfate and urea). Metal ions were not required for its activity, while reducing reagents (beta-Me and dithiothreitol, DTT) increased its activity by 30%.