Fusarium oxysporum f. sp. melonis produces extracellular endo-1,4-beta-xylanase and beta-xylosidase when grown in shaken culture at 26 degrees C in a mineral salts medium containing oat spelt xylan and glucose as carbon sources. Endo-1,4-beta-xylanase was purified 251 times from 5-day-old culture filtrates, by Sephacryl S-200, ion exchange and gel filtration HPLC. The purified sample yielded a single band in SDS polyacrylamide gels with a molecular mass of 80 kDa on electrophoretic mobility and 83 kDa by gel filtration behavior. High activity of the endo-1,4-beta-xylanase against xylan was observed between 5 and 8 pH, and between 40 and 60 degrees C, the optimum pH and temperature being 5.0 and 50 degrees C, respectively. Kinetic properties of the enzyme are similar to those of other fungal xylanases, showing high affinity towards oat spelt xylan with a Km of 1 mM expressed as xylose equivalent.