Two cellobiohydrolases (CBH I and CBH II) were purified from the culture filtrate of the Penicillium occitanis mutant Po16. The molecular weights and isoelectric points of the purified enzymes were 60 kD and 5.2 for CBH I and 55 kD and 5.9 for CBH II. The enzymes differed in amino acid composition and carbohydrate content. They were immunologically unrelated. Yet, both cellobiohydrolases had similar pH optima (4-5), thermal stability, and pH stability (2-9). Maximum activity was at 60 degrees C for CBH I and 65 degrees C for CBH II. Cellobiose competitively inhibited CBH I activity at 5 mM but had no effect on the CBH II activity up to 100 mM. The two cellobiohydrolases acted synergistically toward Avicel and H3PO4-swollen cellulose.