Assessments of the quantity of organic matter in shells were made both by ignition at 475°C and by chemical extraction in O•1M trichloroacetic acid. The quantity of organic matter obtained by ignition at 475°C was over-estimated by 2 to 4•8 times, depending on the species studied (Ruditapes philippinarum, Cardium edule, Cardium glaucum). A technique of extraction by O•lM trichloroacetic acid, filtration on Whatman CFjC filter and weighing of remaining ashes after ignition (450°C) is proposed, Energy values of the shell organic matter varied from 17•00(±0•60) Jmg 1 to 24•0 (±2'87) Jmg 1 depending on the species and the geographic location. In the energy budget, these methods have made it possible to obtain an estimate of the energy mobilised in the production of shell organic matter. Variability in the energy values was the result of variation in the relative proportion of the different biochemical components. The proportion of proteins (Kjeldahl method) varied from 66•7% to 89•7% according to the species and geographical locations. The lipid content varied from 0.84% to 2•88% and carbohydrates from 0•15'70 to 0•29%.