Morphologic and physiologic changes taking place in carbon-limited submerged cultures of Aspergillus nidulans DeltaflbA and fadA(G203R) strains were studied. Loss-of-function mutation of the flbA gene resulted in an altered germination with unusually thick germination tubes, "fluffy" pellet morphology, as well as a reduced fragmentation rate of hyphae during autolysis. In the fadA(G203R) mutant strain, conidiophores formed in the stationary phase of growth, and the size of pellets shrank considerably. There were no significant differences in the generation of reactive oxygen species (ROS) and in the specific catalase and superoxide dismutase activities by the tested mutants and the appropriate parental strains. Therefore, the participation of ROS or antioxidative enzymes in FadA/FlbA signaling pathways seems to be unlikely in submerged cultures. On the other hand, earlier increases in the extracellular-protease and ammonia production were recorded with the DeltaflbA strain, whereas the protease and ammonia production of the fadA(G203R) mutant lagged behind those of the wild-type strains. Similar changes in the time courses of the induction of gamma-glutamyltranspeptidase and the degradation of glutathione were observed. These results suggest that FadA/FlbA signaling may be involved in the mobilization of protein and peptide reserves as energy sources during carbon starvation.