This paper proposes closed loop controls for microorganisms in the chemostat, which make it possible to select species with fastest growth rate in chosen environmental conditions. In particular, by controlling the dilution rate and the input substrate concentration, it is possible to select a species which maximizes a criterion. In a first step a control is proposed for Monod's and Droop's models in order to regulate the total biomass concentration. We show that this causes the selection of the fastest growing species if the system has a periodic behavior. Then we propose a way of achieving periodic stresses, and of selecting the species with maximal potential growth rate. Finally, the method is simulated using Droop's model for selecting a species which can both grow fast and increase its internal substrate storage.