The paper investigates the periodic operation of a continuous bioreactor described by an unstructured model for which the yield coefficient is linearly dependant on the substrate. The model was studied by a number of authors who showed its ability to predict oscillatory behavior for some range of dilution rates. The paper investigates the different dynamic behaviors predicted by the model when the substrate feed concentration is periodically varied. It is shown that when a region of stable limit cycle is forced with a relatively high forcing frequency, the behavior of the process alternates between periodic and complex chaotic behavior via a number of mechanisms of transition, including period doubling and intermittency. It is shown that the time-average performance of the bioreactor can be substantially improved by an appropriate selection of the forcing amplitude. The effect of the position of the center of forcing on the nature of the emerging patterns are also studied. (C) 2010 Elsevier Ltd. All rights reserved.