Cephalosporin C was produced by Acremonium chrysogenum in a stirred tank reactor in fed-batch operation in six different complex cultivation media by using the same composition: with 30 and 50% dilution and 150 and 200% enrichment of the reference 100% medium. The concentrations of the medium components, the beta-lactam precursors and the product were monitored during cultivation. The concentrations, the productivity and the yield coefficients of cephalosporin C were determined. With regard to the yield coefficient the 50% medium and with respect to the productivity the 150% medium seems to be the most economical system. The results of the regulation of CPC production are at odds with previous findings with more primitive strains. (C) 2002 Elsevier Science Ltd. All rights reserved.