L-Ribose and 1.-arabinose were prepared biochemically from ribitol via a two-step reaction, by which the complete oxidation of ribitol to r.-ribulose (~98%) was achieved by the reaction of washed cells of Acetobacler aceti IFO 3281. The produced L-ribulose was then used as a substrate for the production of L-ribose and L-arabinose. The isornerization of L-ribulose to L-ribose and L-arabinose was carried out using L-ribose isomerase(L-RI) of Acinetabacter sp. strain DL-28 and r.-arabinose isomerase (L-AI) of Mycobacterium srnegmatis,respectively. At equilibrium, the ratio of L-l'ib0SB : L-ribulose was 70 : 30 and that of L-arabinose : L-ribulosc was 90 : 10. After a simple puriﬁcation treatment, hnth pentoses could he crystallized without the use of column chromatography. The crystals were conﬁrmed as L-ribose and L-arabinose by High-performance liquid chromatography (I-IPLC), Infrared (IR), Nuclear magnetic resonance (NMR) and optical rotation measurements.