Carbon and nitrogen metabolic budgets and stable isotope analysis were used to determine the fate of the matter assimilated by the suspension-feeding brittle-star Ophiothrix fragilis in summer conditions. Metabolic budgets were established based on the experimental measurements of excretion and egestion rates and on previously published respiration and production rates. Two populations, from sites differing for the availability of macroalgae in the near shores, were investigated to test whether the assimilated matter reflected the local features of available matter. However, C:N (atomic ratio < 6) as well as delta C-13 values (as negative as -25.05 parts per thousand) of the suspended particulate matter indicated a dominance of living microalgae over detrital material at both sites. Furthermore, delta C-13 and delta N-15 values of Ophiothrix fragilis were close for the two sites (respectively -19.56 and -18.62 parts per thousand for delta C-13; 8.29 and 7.40 parts per thousand for delta N-15), supporting the hypothesis of a similar dietary composition for the two populations. Both metabolic budget and stable isotope analysis showed a discrepancy between the assimilated matter and the available matter composition. Some hypotheses are proposed to explain this discrepancy, as well as potential complementary studies needed to test for them.