A series of continuous- and sequencing-batch reactor experiments were performed to assess the growth dynamics of Escherichia coli strain K12-MG1655 in chemostat systems. Previous mathematical predictions and early experimental results had shown that confined oscillatory dynamics ensue in bioreactor populations, which relates to "group birth and death" events within the population. New results are reported here that generally verify the predictions of the model and show that confined oscillations occur under different initial conditions, but the characteristics of the oscillatory dynamics vary as a function of the hydraulic retention time (HRT). Bioreactors were operated at HRTs ranging from 2.7 to 35 h and, regardless of initial conditions or the imposition of transient operational instabilities, highly patterned oscillations developed when HRT was between ∼3 and 8 h. However, outside of this range, bioreactor populations tended to form biofilms on the reactor walls (although the majority of the cells remained suspended in the bulk solution) and stable oscillations were not seen in the bulk phase. This suggests that alternate operating "states" might exist in chemostat populations with biofilm formation and non-homogenous spatial growth influencing "system" dynamics at very low and high HRTs. Although the model accurately predicts a confined dynamic equilibrium for mid-range HRT operations, experimental data show that model predictions do not extend outside of this range, when an alternate stable-state seems to be attained.