Trichoderma species are used commercially as biocontrol agents against a number of phytopathogenic fungi due to their mycoparasitic characterisitics. The mycoparasitic response is induced when Trichoderma specifically recognizes the presence of the host fungus and transduces the host-derived signals to their respective regulatory targets. We made deletion mutants of the tga3 gene of Trichoderma atroviride, which encodes a novel G protein subunit that belongs to subgroup III of fungal G proteins. tga3 mutants had changes in vegetative growth, conidiation, and conidial germination and reduced intracellular cyclic AMP levels. These mutants were avirulent in direct confrontation assays with Rhizoctonia solani or Botrytis cinerea, and mycoparasitism-related infection structures were not formed. When induced with colloidal chitin or N-acetylglucosamine in liquid culture, the mutants had reduced extracellular chitinase activity even though the chitinaseencoding genes ech42 and nag1 were transcribed at a significantly higher rate than they were in the wild type. Addition of exogenous cyclic AMP did not suppress the altered phenotype or restore mycoparasitic overgrowth, although it did restore the ability to produce the infection structures. Thus, T. atroviride Tga3 has a general role in vegetative growth and can alter mycoparasitism-related characteristics, such as infection structure formation and chitinase gene expression.