flbA encodes an Aspergillus nidulans RGS (regulator of G protein signaling) domain protein that is required for control of mycelial proliferation and activation of asexual sporulation. We identified a dominant mutation in a second gene, fadA, that resulted in a very similar phenotype to flbA loss-of-function mutants. Analysis of fadA showed that it encodes the alpha-subunit of a heterotrimeric G protein, and the dominant phenotype resulted from conversion of glycine 42 to arginine (fadA(G42R)). This mutation is predicted to result in a loss of intrinsic GTPase activity leading to constitutive signaling, indicating that activation of this pathway leads to proliferation and blocks sporulation. By contrast, a fadA deletion and a fadA dominant-interfering mutation (fadA(G203R)) resulted in reduced growth without impairing sporulation. In fact, the fadA(G203R) mutant was a hyperactive asexual sporulator and produced elaborate sporulation structures, called conidiophores, under environmental conditions that blocked wild-type sporulation. Both the fadA(G203R) and the fadA deletion mutations suppressed the flbA mutant phenotype as predicted if the primary role of FlbA in sporulation is in blocking activation of FadA signaling. Because overexpression of flbA could not suppress the fadA(G42R) mutant phenotype, we propose that FlbA's role in modulating the FadA proliferation signal is dependent upon the intrinsic GTPase activity of wild-type FadA.