The effect of several carbon sources on the production of alkaline phosphatase by the thermotolerant Aspergillus caespitosus was analysed. The fungus released high levels of alkaline phosphatases into the medium after being cultured for long periods with xylan or industrial residues such as wheat raw and sugar cane bagasse in the culture media. In contrast, the alkaline phosphatase activities were found only intracellulary when the fungus was cultured in glucose-supplemented media. The pH of the medium likely affects the process of enzyme secretion according to the carbon source used. Addition of xylan or industrial residues in the culture medium stimulated the secretion of phosphatases. In contrast, media supplemented with glucose or disaccharides promoted retention of these enzymes into the cells. The subcellular location activities of alkaline phosphatases were studied using histochemical and immunochemical methods and showed that alkaline phosphatases were present in the mycelial walls and septa.