The energy density (E-D) of anchovy Engraulis encrasicolus in the Bay of Biscay was determined by direct calorimetry and its evolution with size, age and season was investigated. The water content and energy density varied seasonally following opposite trends. The E-D g(-1) of wet mass (M-W) was highest at the end of the feeding season (autumn: c. 8 kJ g(-1) M-W) and lowest in late winter (c. 6 kJ g(-1) M-W). In winter, the fish lost mass, which was partially replaced by water, and the energy density decreased. These variations in water content and organic matter content may have implications on the buoyancy of the fish. The water content was the major driver of the energy density variations for a M-W basis. A significant linear relationship was established between E-D g(-1) (y) and the per cent dry mass (M-D; x): y = -4.937 + 0.411x. In the light of the current literature, this relationship seemed to be not only species specific but also ecosystem specific. Calibration and validation of fish bioenergetics models require energy content measurements on fish samples collected at sea. The present study provides a first reference for the energetics of E. encrasicolus in the Bay of Biscay.