ALTHOUGH there is considerable morphological similarity between different species of freshwater snail in terms of general body form and radular construction1, differences in diet occur2,3. Furthermore, preferred food materials are invariably digested more efficiently than less preferred materials4 (Fig. 1). The implications are that differences in diet and resultant niche separations are dependent on physiological, rather than morphological, divergence and that the former is probably related to the quality and quantity of enzyme secretion. Thus it may be possible to reconcile the conflict first noted by Boycott5 between the general observation that most species of freshwater gastropod can and do coexist5,6 and the so-called "competitive exclusion principle". To test this hypothesis we have examined cellulase activity in 14 species of snail and have linked our results to known dietary differences and to direct observations on assimilation efficiencies. We have found a strong correlation between enzyme activity and assimilation efficiency, and that dietary preferences can be ascribed to physiological differences of this kind.