Modeling of hyaluronic acid (HA) production by fermentation of Streptococcus zooepidemicus was performed. An accurate determination of the growth curve of the cocoid cells was first discussed, which was relevant to the change of cell morphology after entry into the stationary phase. A delayed growth-associated model was then proposed for the HA production, which was in accordance with mechanisms of HA formation reported in the literature. The proposed model was employed satisfactorily not only for HA production, but also for poly-gamma-glutamic acid production. (C) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.