Fermentations utilizing genetically modified microbes require accurate monitoring and control. This is done through a control algorithm based on a process model; control usually involves continual measurements and manipulations of the substrate feed rate and its concentration. Since variations in the data, due to the limitations on measurements or external influences, affect the performance, it is useful to have a quick evaluation of these effects before sensitivities or control strategies are studied in detail. Condition numbers provide a convenient way to do this. They have been employed here for a continuous fermentation for beta-galactosidase production by Saccharomyces cerevisiae containing the plasmid pSXR125. When all recombinant cells have the same number of copies of the plasmid, a selective medium permits less precision in measurement and manipulation than a non-selective medium, but this is reversed when there are two sub-populations with different numbers of plasmids. The more stringent requirement for a selective medium in the latter case is, however, offset by its greater productivity.