Abstract. The ecological role of foraminifers has been largely unknown partly owing to difficulties in determining their individual biomass, although foraminifers are abundant in surface marine sediments. The present study provides a reliable and inexpensive method for the quantification of the protein content of hard-shelled foraminifers as a measure of biomass while preserving the tests for later analyses (e.g. morphometry, stable isotopes), using nano-spectrophotometry. The protein biomass, is significantly correlated with size, and shell weight of Ammonia tepida (n = 102, p < 0.00001, R² = 0.462, and n = 181, p < 0.00001, R² = 0.855). Variability in the relation between test size and weight, and cell biomass may result from natural variability in horizontal and vertical microenvironments encompassing metabolic state, as well as variability in test morphometry and calcite mass (i.e. test weight). In turn, knowing the size- and species-specific biomass of foraminifers adds valuable information on the trophic and ecologic conditions of modern and ancient marine environments, in particular on the reconstruction of the regional palaeoproductivity and flux of organic matter.