Cephalosporin C Fermentation was studied using Cephalosporium acremonium mutant strain, which was isolated after U.V. mutagenesis of Cephalosporium acremonium ATCC 14553. It produced about 520 ug Cephalosporin C/ml-broth in complex media and 230 ug Cept. C/ml-broth in synthetic media in shake flask fermentation. We tested the effect of methionine concentration on Cephalosporin C production to find the optimal condition and it was also tried to investigate the kind and amounts of substances that would stimulate production of the antibiotics; 1\% oleic acid increased the productivity by 20\%, 0.15\% calcium chloride by 20\%, and the combination of 0.7\% ammonium sulfate and 1.0\% calcium carbonate by 75\%. The effect of pH on Cephalosporin C productivity also of some interest was studied. It was shown that Cephalosporin C is acid-stable and alkaline sensitive, and much deactivation occurred above pH 8.0. Fermentation variables have been studied for further understanding of the fermentation process. Such variables as suger concentration, pH, specific oxygen uptake rate, antibiotic concentration, and dry cell weight were measured. Generally antibiotic formation was delayed till the completion of growth phase, and many possible mechanisms have been suggested for this control of antibiotic biosynthesis. One of the most plausible mechanisms, the role of glucose on the biosynthesis of Cephalosporin C and pencillin N was investigated quantitatively. The presence of glucose inhibited the synthesis of these antibiotics: The specific Cephalosporin C productivity was 189.6 ug Ceph. C/g D.C.W./hr when 2\% glucose was used, 71.0 in case of 8\% glucose, and 50.2 in case of 16\% glucose. Based on these data and the fermentation study where glucose was fed, it was concluded that the inhibition of Cephalosporin C production is due to catabolite inhibition by glucose in the fermentation media.