This chapter addresses the control of microbial specic growth rate in fedbatch bioreactors. Practical constraints concerning the available signals and information on the process are dealt with. Then, for a large class of bioreactions, namely pure cultures with one limiting substrate and oxygen in excess, the control problem is regarded as a problem of coordinating control involving the invariance and attractivity of nontrivial sets in state space. Both monotonous and non-monotonous kinetic functions are dealt with. The dissolved oxygen signal can give valuable information about the cell culture, when the oxygen supply is in balance with the consumption. This is utilized for a probing feeding strategy, treated in chapter 1, part II.