This study reports on the effects of growth temperature on the secretion and some properties of the xylanase and beta-xylosidase activities produced by a thermotolerant Aspergillus phoenicis. Marked differences were observed when the organism was grown on xylan-supplemented medium at 25 degrees C or 42 degrees C. Production of xylanolytic enzymes reached maximum levels after 72 h of growth at 42 degrees C; and levels were three- to five-fold higher than at 25 degrees C. Secretion of xylanase and beta-xylosidase was also strongly stimulated at the higher temperature. The optimal temperature was 85 degrees C for extracellular and 90 degrees C for intracellular beta-xylosidase activity, independent of the growth temperature. The optimum temperature for extracellular xylanase increased from 50 degrees C to 55 degrees C when the fungus was cultivated at 42 degrees C. At the higher temperature, the xylanolytic enzymes produced by A. phoenicis showed increased thermostability, with changes in the profiles of pH optima. The chromatographic profiles were distinct when samples obtained from cultures grown at different temperatures were eluted from DEAE-cellulose and Biogel P-60 columns.