The use of 4-methylumbelliferyl-β-D-glucuronide (MUG) in different solid media for the detection and enumeration of Escherichia coli in foods was evaluated by testing the effects of different substrate concentrations (50 or 100 μg ml−1), incubation temperatures (37 or 41·5°C) and incubation times (8, 12, 24 and 48 h). Different kinds of foods, both naturally and artificially contaminated, were analysed. The use of selective media without differential substances and an incubation time of 24 h seem to be worthy of recommendation. In this case an incubation temperature of 37°C would be preferred and the MUG concentration could be reduced to 50 μg ml−1. Incubation times shorter than 24 h, which may cause a loss of sensitivity, require higher incubation temperatures (41·5°C) and MUG concentration (100 μg ml−1).