SummaryA model for the growth of an organism on multiple substrates was developed, assuming that. each substrate has a competitive inhibition effect, on the uptake of other substrates. The model was extended to examine mixed substrates, showing that the coexistence of several species a t steady state in continuous cultures is possible, even when all the organisms all strongly prefer the one substrate. The diversity of nutrient sources in a real system may be a key factor in supporting a heterogeneous microbial population.