Giant clams form a symbiosis with photosynthetic algae of the genus Symbiodinium that reside in clam mantle tissue. The allometry of symbiont photosynthetic performance was investigated as a mechanism for the increasing percentage of giant clam carbon respiratory requirements provided by symbionts as clam size increases. Chlorophyll fluorescence measurements of symbionts of the giant clam Tridacna maxima were measured during experiments conducted in September of 2009 using specimens 0.5–200 g tissue wet weight (3–25 cm long), collected from waters around southern Taiwan (N 21o 36', E 120o 47') from July to August of 2009. Light-dependent decreases in effective quantum yield (DF/Fm' 0) calculated as the noontime maximum excitation pressure over PSII (Qm), relative electron transport rates (rETR), and darkadapted maximum quantum yield (Fv/Fm) all varied as a quadratic function of clam size. Both Qm and rETR increased as clam size increased up to ~10–50 g then decreased as clam size increased. Fv/Fm decreased as clam size increased up to ~5–50 g then increased as clam size increased. Chlorophyll fluorescence measurements of rETR were positively correlated with gross primary production measured during chamber incubations. Overall, symbionts of mid-sized clams ~5–50 g exhibited the highest light-dependent decreases in effective photosynthetic efficiencies, the highest relative electron transport rates, and the lowest maximum photosynthetic efficiencies, and symbiont photosynthetic performance is allometric with respect to host clam size.