The xylanolytic system is produced mainly by microorganisms, among these, filamentous fungi are important, because they secrete enzymes into the medium and their xylanolytic levels are much higher than those found in yeast and bacteria. Xylanolytic enzymes serve for many industrial applications, such as animal feed, bakery, textiles, cellulose pulp and paper. The main xylanolytic enzyme is endo-1,4-b-xylanase (1,4-b-D-xylan xylanohydrolase; EC 3.2.1.8) that catalyzes the endohydrolysis of 1,4-b-D-xylosidic linkages in xylan. The aim of this work was to select a fungus with good production of endoxylanase, and to determine some biochemical parameters of the enzyme. Endoxylanase activity was determined using Birchwood xylan as substrate. The reducing sugar released was quantified with 3,5 dinitrosalicylic acid (DNS). Aspergillus niger van Thieghem was selected among 28-fungi screened. SR medium was the most convenient to produce xylanases. This medium induced about 35% more xylanase than Khanna medium, frequently used for xylanase production. The best conditions for endoxylanase production were found with cultures grown at 40ºC for 72 hours, under standing conditions, with 1% "Birchwood" xylan as carbon source. The temperature and pH optima of the xylanolytic crude extract were 65ºC and 2.5, respectively. The enzyme was stable for one hour at 60ºC. At 65ºC it exhibited a t₅₀ of approximately 70 minutes. PEG (10% final concentration) slightly protected (10-15%) the enzyme from inactivation, but glycerol, sorbitol or trehalose were without effect. These characteristics of the A. niger endoxylanase, such as high temperature optimum, thermostability, and pH stability, suggest a possible practical application for this enzyme. Financial support: FAPESP, CAPES and CNPq.