Cephalosporin production by Streptomyces clavuligerus is regulated by some type of carbon catabolite control. Increasing concentrations of preferred carbon sources, such as glycerol and maltose, decreased production of the antibiotics. Poorer carbon sources, such as α-ketoglutarate and succinate, led to high specific production of cephalosporins and shifted the dynamics of fermentation to a greater degree of association with growth. The results support the concept that the phase in which a product is made by a microorganism is not a function of the particular molecule produced, but rather of the nutritional environment presented to the organism.