Transient or dynamic responses often occur in continuous aerobic systems like the activated sludge process when environmental variables change. The system's response to a dynamic load change has yet to be successfully modeled. In this paper we examine previously collected experimental data of the system's response to a step change. The real time profile of the growth rate mu and the cell/substrate yield, Y, are studied via parametric manipulation. It was observed that both variables, mu and Y, experience an initial step drop and then a hysteresis response to a new steady-state level, following a sudden step change in the feed flow rate. A modified Monod equation is proposed here to qualitatively model the hysteresis dynamic response of a sudden dilution rate change to an open, continuous microbial growth system.