A general model is presented for recombinant fermentation in a chemostat in which micromixing may be imperfect. Using the production of tryptophan synthetase as an example and the dilution rate,D, as the bifurcation parameter, conditions are first derived for Hopf bifurcation in a perfectly mixed chemostat. The bifurcation surfaces in three-dimensional space are such that oscillatory solutions exist above the surfaces and washout conditions below. A similar analysis is done for imperfect mixing. The bifurcation region is a concave hyperboloid in the [D,f, n]-space, wherefandn; are model parameters. It is argued that poor macromixing reduces the range ofDover which bifurcation to periodic solutions is possible.