Previous studies have shown that when two species compete for the same substrate, a controlled degree of segregation in a bioreactor favours growth of the desired species. That concept has been extended to the growth of bacteria on a mixture of two substrates for one of which the organism has a natural preference. However, using the example of Escherichia coli cultivated in a medium containing glucose and fumarate, it has been shown that, while the physiological preference for glucose is maintained, incomplete macromixing promotes growth and enables better utilisation of both substrates. While the preferred substrate (glucose) is consumed first in a perfectly mixed reactor, imperfect mixing allows both substrates to be utilised simultaneously, at different rates. By dividing the broth conceptually into an inoculated region and a non-inoculated region, it is seen that cell growth is favoured by an initial period of good mixing followed by poor mixing in the non-inoculated region.