Cellulase hyperproducing mutants derived from the fungus Trichoderma reesei QM9414 were analyzed. They exhibited higher filter-paper degrading activity and a lower growth rate than the wild-type QM9414 strain. Transcription of the cellobiohydrolase I and endoglucanase I genes in the mutants was also greater than that of QM9414, suggesting that cellulase hyperproduction by these mutants was regulated at the transcriptional level.