Phenotypic flexibility in food selection and digestive efficiency in response is examined in relation to variations in food quality in a detritivorous species of Mediterranean ecosystems, the woodlouse Porcellio laevis Latreille (Isopoda: Oniscidea). It is hypothesized that diet selection should show a positive correlation between diet quality and digestive efficiency. The studied plants are isocaloric but exhibit significant differences in terms of nitrogen, carbon and fibre content. The observations appear to support the hypothesis that these isopods show compensatory feeding behaviour when feeding on poor diets. Contrary to expectation, a dietary effect on relative growth rate is not observed. It is concluded that behavioural skills associated with diet selection and physiological flexibility may allow P. laevis to satisfy and maintain equivalent nutritional levels during exposure to different diet qualities. In general, it appears that physiological flexibility plays an important role in determining nutritional balance in P. laevis, which can be of great benefit in the highly seasonal environment that it inhabits.