In Trichoderma harzianum CECT 2413 external pH regulates essential functions such as growth rate, sporulation, cell and colony morphology, pattern of secreted proteins, gene expression or mycoparasitism-related enzymatic activities. pH regulation is mediated by the transcriptional factor Pac1 (homologous to PacC regulator in other fungi), encoded by pac1 whose expression increases with pH. Two pac1 mutants have been obtained from CECT 2413: P2.32, that possesses an allele of pac1 active at any pH, and R13, that does not express pac1 due to interferent RNA. Cell morphology, sporulation and growth rate are optimal for strain P2.32 at pH 7.5 and for strain R13 at pH 3.0, mimicking alkaline and acidic conditions, respectively. Pac1 regulates some genes involved in antagonism: chit42 chitinase, papA protease, gtt1 glucose permease, and qid74 cell wall protein. Furthermore, Pac1 modulates T. harzianum antifungal activity since wild type and mutants inhibit several phytopathogenic fungal strains at various degrees in different assays.