We investigate a bioreactor cascade consisting of two reactors. For a given total residence time, we study how the performance of the reactor (measured either as the cell mass concentration or the reactor productivity) depends upon the feed substrate concentration and the residence time in the first reactor. The bioreactor model in this study uses a growth rate that is given by a Monod expression with a yield coefficient that is a linear function of the substrate concentration. Previous researchers have compared the performance o f a two-reactor system against a single reactor with the same total residence time. The main focus of this paper is to show that the performance o f a two-reactor cascade should not be gauged in this manner, as comparisons using this criterion can give grossly misleading results. Our analysis shows that before maximising the performance o f a cascade, we must first consider the performance o f a single reactor system as a benchmark.