This study compared suspension f e e d n g , assimdation efficiency, respiration and excretion, and energy budgets (= scope for growth, SFG) in relation to body size in 2 pearl oysters, Pinctada maryaritifera and P. maxima, at a low food concentration (ca 5000 cells ml-l Tahitian Isochrysis galbana). Clearance rate (CR), respiration rate (K) and ammonia excretion rate (E) were strongly correlated with body size ( p < 0 001) in both species, wlth exponents of 0.60 and 0.61 (CR), 0.44 and 0.56 (R), and 0.64 and 0.78 (E), respectively, for P margaritdera and P maxima. CR did not differ significantly between the species, but absorption efficiency, which was unrelated to size, was significantly greater in P maxima (57 5 vs 51 %, p < 0.05). There was, however, no significant difference in absorbed energy (AE) between the species. Respired energy (RE) and excreted energy (EE) as proportions of AE were slgniflcantly lower ( p < 0.01) in P. maxima of 0.1 g dry soft tissue wt (ca 36 mm shell height, SH). The former was 0.36 compared to 0.58 in P. margaritifera of the same size. Thus, P. m a x m a of 0.1 g dry soft tissue wt exceeded P. margaritlfera of the same size in SFG, which accords with the former species' more rapid early growth. Both species of pearl oysters have a high abihty to acquire energy under low phytoplankton conditions. Both species are exceptional bivalves in terms of energy fluxes, with clearance rates of 50 to 100 1 h-' in large oysters of 150+ mm SH. They show among the highest CR, R, E and SFG values recorded for blvalves (using 1 g dry soft tissue wt as a standard size). The largest g a n t clam, Tridacna gigas, is one tropical bivalve with comparable SFG. It, however, 1s dependent on energy from autotrophy as well as heterotrophy to achieve its high SFG.