The ascomycete Trichoderma reesei is known to produce a variety of cellulases and hemicellulases and the hyper-cellulolytic mutants of this fungus are useful as industrial cellulase producers. In Japan, PC-3-7, derived from the early mutant QM9414, is well-known as a cellulase hyperproducing mutant. In addition to the productivity of enzymes, the composition of secreted enzymes greatly influences biomass saccharification. Therefore, we evaluated the cellulase productivity of T. reesei mutants in Japan at different pH as a factor influencing enzyme production. At higher pH values, QM9414 exhibited reduced cellulase productivity whereas PC-3-7 maintained high cellulase productivity and gene expression at the transcriptional level. The gene encoding the pH-responsive transcription factor PACI did not mutate in PC-3-7, and its expression pattern against different pH conditions was similar between QM9414 and PC-3-7. Furthermore, the deletion of pac1 encoding PACI caused different expression patterns of cellulase genes between QM9414 and PC-3-7. Therefore, we suggest that T. reesei possesses a pH-responsive cellulase production mechanism that is different from the PACI-related mechanism. Finally, we identified that N-25, a strain developed at an early stage of mutant development acquired cellulase productivity at a higher pH. In this investigation, we also found and tested candidate genes possibly affecting pH response using comparative genome analysis.