Cellulase production by a hypercellulolytic mutant Pol6 of Penicillium occitanis was studied in pure culture and also using a mixed culture of P. occitanis and Trichoderma reesei. Although cell yields were equivalent, the cellulase production was much higher when using only P. occitanis. Mixed culture showed similar enzyme production to that of P. occitanis but after a longer culture period. Pure and mixed filtrates of P. occitanis and T. reesei cultures were examined and compared for their efficiency in degrading esparto grass cellulose, a local paper waste. Using roughly similar filter paper activities of both filtrates, that from P. occitanis seemed to be particularly effective against this substrate. No synergistic activity was apparent between P. occitanis and T. reesei cellulases.