For bioprocess optimization, plug-flow beheviour is an option in many cases. However, many bioprocesses require oxygen to be supplied into the bioreactor. Plug-flow-like behaviour with aeration is feasible in a series of bioreactors. This article reviews the range of aerated bioreactors available for the approximation of plug-flow behaviour and introduces a new type of serial air-lift bioreactor. The optimal design of cascades of serial bioreactors is also discussed.