For a better understanding of the role of mesopelagic fish in the Southern Ocean food web, the energy and water content of Bathylagus antarcticus, Electrona antarctica and Gymnoscopelus braueri from the Lazarev Sea were investigated. Mean dry weight energy content of B. antarcticus (20.4 kJ g(-1)) was significantly lower than in E. antarctica and G. braueri (both 29.4 kJ g(-1)). In E. antarctica, an increase of dry weight energy density with age was evident from 26.9 kJ g(-1) in juveniles of less than 1 year of age to 32.0 kJ g(-1) in 3-year-old fish. Water content decreased with size in all three species. Abundant high-energy species such as E. antarctica are at a key position in the food web. Due to a marked influence of age on energy content, population structure can be an important variable in estimates of energy fluxes in the Southern Ocean ecosystem.