A total of 506 isolates of mesophilic, thermophilic and thermotolerant fungi isolated from the poultry feed ingredients included soybean meals, ground maize, cottonseed cake, wheat bran and fish meal, on glucose-Czapek's agar, Littman oxgall agar at 28 degrees C and yeast starch agar (YPSs) at 45 degrees C, were screened for their ability to produce hydrolytic protease enzyme on solid media. Most of the fungal isolates were able to produce such enzymes but with variable capabilities. The highest proteolytic activity was exhibited by some isolates of Penicillium chrysogenum, Aspergillus flavus, Thermoascus thermophilus and Rhizopus chizopodifarmis. Of all fungal isolates screened for proteolytic activity, Penicillium chrysogenum and Thermoascus thermophilus produced the highest amounts of proteases. These two isolates were used to study the effect of some environmental and nutritional factors on their proteolytic activity. It was found that the highest yield of protease by P. chrysogenum (12.5 units) was achieved 3 days after incubation at 30 degrees C. Marked reduction in protease activity was observed at 37 degrees C. The thermophilic fungus T. thermophillus exhibited maximum (18 units) proteolytic activity 6 days after incubation at 45 degrees C. The enzyme yield was reduced to 13 units at 50 degrees C. Among the seven carbon sources tested, sucrose was the most appropriate for maximum protease production by both P. chrysogenum and T. thermophilus (13.2 and 12.8 units, respectively). Of the sixteen nitrogen sources investigated, NaNO3 was the best inorganic additive nitrogenous salt which induced the highest proteolytic activity by P. chrysogenum and T. thermophilus, whereas DL-tryptophan was the most preferable organic nitrogen compound for maximum protease production by the two fungi tested.