In this work, we considered a multistage integrated continuous fermentation process for producing lactic acid. Each stage consists of a mixing tank, a bioreactor, a cell-recycle unit, and an extractor. A generalized mathematical model was formulated to express the integrated process. We have compared the overall productivity and conversion of the integrated process with those of two simplified processes. From the design equations, three processes have an identical overall conversion. However, the proposed process has the greatest overall productivity. A specific kinetic model for lactic production was applied to the integrated process to formulate it as a flexible or fuzzy optimization problem to find the maximum overall productivity and conversion with interval residual and supplied glucose constraints. Four optimization problems were considered to determine the optimal stages, operating conditions, and design variables. If the decision variables excluded the working volume ratio and flow rate ratio in the optimization problem, four stages were required to yield the maximum overall productivity and near complete overall conversion. However, if both operation variables were considered as the decision variables in the optimization problem, three stages were sufficient to achieve the identical overall productivity. (c) 2007 American Institute of Chemical Engineers.