The highest possible dilution rate at which steady state is able to be attained within a constant volume bioreactor such as a fermenter with continuous inflow and outflow. The dilution rate is the ratio of the flow of fresh feed to the volume of broth containing a viable population of microbial cells. At the critical dilution rate, the rate (p. 87) of microbial growth is insufficient to replenish the cells being washed out. Above the critical dilution rate, all the cells are eventually washed out.