This study aimed to determine the influence of microalgal species and food concentration=520 \\ on various physiological parameters and Scope for Growth (SFG) in adults of 2 pearl oysters, Pinctada=520 \\ margantifera and P maxima. Clearance rate, pseudofaecal production rate, absorption efficiency, respiration=520 \\ rate and excretion rate were determined over a range of food concentrations using 2 microalgal=520 \\ diets, Tah~tian Isochrysis sp. (T-Iso) and Dunaliella primolecta at 28'C. Clearance, pseudofaecal production=520 \\ and respiration rates were significantly affected by microalgal diet. From these results, and=520 \\ because of the higher energy content of T-Iso, pearl oysters feeding on T-lso had maximum values of=520 \\ SFG that were 1.5 to 2 1 times higher than when feeding on D. pnmolecta. Clearance rate and absorption=520 \\ efficiency were significantly related to food concentration as negative exponential relationships=520 \\ (p c 0 001). Generally, pseudofaecal production, respiration and excretion rates were significantly=520 \\ related to food concentration as positive linear relationships (p < 0.005) Optimal food concentrations=520 \\ for max~mumS FG for P margarjtifera and P maxima were 1 to 2 mg I-' and 2 to 3 mg I-', respectively.=520 \\ P. maxima was better adapted to a wider range of food concentrations. P. maxima maintained positive=520 \\ SFG up to 9 mg I-' food concentration when feeding on T-lso and up to 7 mg I-' when feeding on D.=520 \\ primolecta, while equivalent values for P. margaritifera were 7 mg 1-' and 5 mg I-', respectively. Thesc=520 \\ results are in accordance with P. maxin~a occurring in a wider range of habitats than P margaritiferd,=520 \\ and experiencing greater concentration ranges of suspended particulate matter