A three dimensional microbial continuous culture model with a restrained microbial growth rate is studied in this paper. Two types of dilution rates are considered to investigate the dynamic behaviors of the model. For the unforced system, fold bifurcation and Hopf bifurcation are detected, and numerical simulations reveal that the system undergoes degenerate Hopf bifurcation. When the system is periodically forced, bifurcation diagrams for periodic solutions of period-one and period-two are given by researching the Poincar e map, corresponding to different bifurcation cases in the unforced system. Stable and unstable quasiperiodic solutions are obtained by NeimarkSacker bifurcation with different parameter values. Periodic solutions of various periods can occur or disappear and even change their stability, when the Poincar e map of the forced system undergoes Neimark-Sacker bifurcation, flip bifurcation, and fold bifurcation. Chaotic attractors generated by a cascade of period doublings and some phase portraits are given at last. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.5000152]The microbiological fermentation technique is widely applied in many fields for its economic importance. It is also investigated due to the complex behaviours observed during the process. We study bifurcations of the microbial continuous culture model with two types of dilution rates: the steady dilution rate and periodically forced dilution rate. For the steady dilution rate, we prove that the model undergoes fold bifurcation and Hopf bifurcation. When the dilution rate is periodically forced, we find that the bifurcations of the equilibria of the unforced system can be extended to the forced system as bifurcations of periodic solutions. Furthermore, periodic perturbation can give rise to complex dynamics, such as quasiperiodic solutions, periodic solutions of various periods, and chaos. In addition, the various periodic solutions can well explain the oscillation phenomena observed in laboratory experiments.