Many of the early studies of ecologica l energetics dealt with aquatic ecosystems (Odum & Odum 1955; Odum 1957; Teal 1957; Richman 1958;) and Engelmann (1966) suggested that this may be due to the more complex nature of terrestrial communities. However, from about 1960 until the commencement of the International Biological Programme, energy studies have mostly been made at the population rather than ecosystem level, and it is aquatic populations that have been relatively neglected. This is perhaps related to the difficulties encountered in sampling populations in freshwater habitats. The aim of the present study was to produce a complete energy budget for a population of Asellus aquaticus by combining field population data with studies of respiratory and feeding activity in the laboratory. The site selected for this study was Wistow Lake, near Leicester, England.