Article révisé par les pairs
Résumé : When a population of micro-organisms giving rise to non-reverting mutants is allowed to grow in a chemostat and when the mutation rate is assumed to be proportional either to the growth rate or to absolute time, three different final situations will occur. The first one is that both populations are washed out because the flow rate was badly chosen (too fast). The second is that the mutant having a selective advantage replaces the normal population in the culture vessel. The third possibility is that both populations grow simultaneously and that between their respective concentrations, a constant ratio gets established. We have studied this last case in detail and have shown the importance of the flow rate in the establishment of a stable ratio. © 1967.