Oscillations in continuous microbial cultures are well documented for baker’s yeast. The oscillations can be modeled by unstructured population balance models in which a key cell cycle parameter, such as the division age, is a function of the substrate or product concentration. The state variables of the periodic solutions exhibit average values different from those of steady state cultures. This suggests that oscillating cultures can be used to optimize microbial processes. Models analyzed here indicate that the possible improvements of oscillating cultures over steady state cultures are insignificant when the cultures are harvested continuously but not when harvested periodically.