An energy budget has been evaluated for a natural population of Crepidula fornicata living at low water of spring tides in the oyster farming basin of Marennes-Oleron. This budget of production was estimated according to the position along the chain. The average biomass for an individual representative of the population was equivalent to 1.87 Kjoules, and the annual consumption C was 20.08 Kjoules/year. The energy budget expressed in percent of energy consummates was: C = Pg + Pr + Ps + R + F; 100 = 4.6 + 1.6 + 0.8 + 48 + 45. This budget is compared with literature data on other herbivorous gastropods and with those calculated for Crassostrea gigas in the same locality.