An agar-degrading Thalassomonas bacterium, strain JAMB-A33, was isolated from the sediment off Noma Point, Japan, at a depth of 230 m. A novel alpha-agarase from the isolate was purified to homogeneity from cultures containing agar as a carbon source. The molecular mass of the purified enzyme, designated as agaraseA33, was 85 kDa on both SDS-PAGE and gel-filtration chromatography, suggesting that it is a monomer. The optimal pH and temperature for activity were about 8.5 and 45 degrees C, respectively. The enzyme had a specific activity of 40.7 U/mg protein. The pattern of agarose hydrolysis showed that the enzyme is an endo-type alpha-agarase, and the final main product was agarotetraose. The enzyme degraded not only agarose but also agarohexaose, neoagarohexaose, and porphyran.