A microelectrode system is used in order to simultaneously measure pH and oxidoreduction potential (Eh) gradients developed during growth by Escherichia coli and Lactobacillus plantarum immobilized in gelled media used as model food. Unlike E. coli, L. plantarum steadily decreased medium pH independently of depth of measurement and time of incubation. Both bacteria brought about the creation of an Eh gradient throughout the gelled medium. This gradient was much more important for E. coli (700 mV) than for L. plantarum (80 mV) but more transitory.