A cyclic AMP (cAMP)-dependent protein kinase pathway has been shown to reg-ulate growth, morphogenesis and virulence in filamentous fungi. However, the precise mechanisms of regulation through the pathway remain poorly understood. In Neurospora crassa, the cr-1 adenylate cyclase mutant exhibits colonial growth with short aerial hyphae bearing conidia, and the mcb mutant, a mutant of the regulatory subunit of cAMP-dependent protein kinase (PKA), shows the loss of growth polarity at the restrictive temperature. In the present study, we isolated mutants of the catalytic subunit of the PKA gene pkac-1 through the process of repeat-induced point mutation (RIP). PKA activity of the mutants obtained through RIP was undetectable. The genome sequence predicts two distinct catalytic subunit genes of PKA, named pkac-1 (NCU06240.1, AAF75276) and pkac-2 (NCU00682.1), as is the case in most filamentous fungi. The results sug-gest that PKAC-1 works as the major PKA in N. crassa. The phenotype of the pkac-1 mutants included colonial growth, short aerial hyphae, premature conidiation on solid medium, inappropriate conidiation in submerged culture, and increased thermotolerance. This phenotype of pkac-1 mutants resembled to that of cr-1 mutants, except that the addition of cAMP did not rescue the abnormal morphology of pkac-1 mutants. The loss of growth polarity at the restrictive temperature in the mcb mutant was suppressed by pkac-1 mutation. These results suggest that the signal transduction pathway mediated by PKAC-1 plays an important role in regulation of aerial hyphae formation, conidiation, and hyphal growth with polarity.