SUMMARY: Two distinct forms of growth of wild-type Neurospora crassa can be produced by altering the nitrogen source of the growth medium. One form completely lacks conidia and carotenoids; the other exhibits enhanced conidiation and pigmentation. Analysis of the oxidative and glycolytic metabolism of these two forms showed that the non-conidiating cultures had significantly greater glycolytic activity than the conidiating cultures, as measured by the production of ethanol, the presence of ethanol dehydrogenase and pyruvate decarboxylase activity and the response of the two culture types to glycolytic inhibitors. When glycolysis was inhibited, conidiation occurred. When oxidative metabolism in normally conidiating cultures was interfered with conidiation was suppressed. These results suggest that the relative activities of fermentative and oxidative pathways in the cell regulate both conidiation and pigment synthesis. The most probable point at which this control is exercised is at the stage of pyruvate, the common branch point of these two pathways. Conidiating cultures also were found to have very high NADP nucleotidase activity, while purely mycelial cultures were essentially devoid of this enzyme. An electron microscope examination of these two culture types revealed differences in cell-wall composition, in the distribution of osmiophilic granules within the cytoplasm and in mitochondrial structure.