Locomotion in the Pulmonata may have two, distinct, negative effects on total metabolism. Energy losses may occur via mucus secretions and also as a result of muscular and ciliary activities. These aspects of pulmonate metabolism were investigated in two freshwater species; Ancylus fluviatilis and Planorbis contortus. The former is a herbivore which feeds on periphyton and the latter is a detrivore which appears able to make use of bacteria. Speed of movement was influenced by starvation time, food availability, and water movement, although the extent to which these factors affected locomotion showed some variation with species. In still conditions P. contortus moved more rapidly than A. fluviatilis whereas the converse occurred under conditions of more violent water movement. During deprivation, however, both species showed similar responses in that their rate of locomotion increased to some maximum and then fell, and their pattern of movement became more random. These strategies were explained in terms of fitness. Under conditions of satiation locomotion had a measurable quantitative effect on P. contortus only, and contributed ca. 20% to routine metabolism. Nevertheless, under conditions of starvation locomotion influenced the respiratory metabolism of both species and in this case there were clear differences between the respiratory rates of constrained and non-constrained subjects. These results were used to explain certain differences recorded by earlier workers on the relationship between starvation and respiration in freshwater snails. Energy losses via mucus secretions were found to make a significant contribution to snail energetics, representing between 13-32% of the energy absorbed across the gut wall. However, the potential energy thus released may not be completely lost from the secreting system because mucus was found to stimulate the growth of those bacteria which are preferentially ingested by P. contortus. Consequently, mucus may play a provendering role and should not be ignored as a vehicle of biologically useful energy in aquatic or terrestrial ecosystems.