A gene, xynX, encoding a novel xylanase, was cloned from Aeromonas caviae ME-1. This gene encoded an enzyme that was constituted of 334 amino acid residues (38,580 Da) and was similar in sequence to Family 10 (Family F) beta-1,4 endo-xylanases. XynX produced only xylobiose and xylotetraose from birch wood xylan, and xylotriose, xylopentaose, and higher oligosaccharides were not detected in the TLC analysis. We designated it as X2/X4-forming xylanase. This enzyme does not have transglycosylation activity. These data suggested that this enzyme is a possible exo-xylanase. According to homology modeling, the enzyme has a ring-shaped (alpha/beta)8 barrel (TIM barrel) structure, typical of Family 10 endo-xylanases, with the extraordinary feature of a longer bottom-loop structure.