Aquaculture of Penaeus vannamei in low-salinity ponds is a growing industry in the southeastern United States. A major challenge facing this industry is a phenomenon called late-term mortality. This is thought to be driven by thermal stress at the end of the growing season when water temperatures can reach or even exceed 36 ºC in shrimp production ponds. In this study, we test whether thermal tolerance decreases with increasing shrimp age/size, and whether aerobic scope (AS) is a useful concept for understanding the physiological basis of thermal tolerance in shrimp. Aerobic scope represents the excess capacity of an organism to deliver oxygen in support of activity, growth, and reproduction, and exhibits a unimodal relationship with temperature. In theory, the temperature at which AS = 0 represents the upper thermal limit of an organism where it is no longer physically capable of meeting its energetic needs. We exposed two size-classes (2.2 ± 0.8 and 25.8 ± 2.4 g) of shrimp to increasing temperature at a rate of 1 °C/h from 20 - 42 °C. At each temperature, we used intermittent respirometry to estimate resting metabolic rate (RMR) and the electron transport system (ETS) assay to estimate maximum metabolic rate (MMR). We directly measured thermal tolerance in terms of critical thermal maximum (CTM): the temperature at which shrimp lost equilibrium and were no longer able to right themselves. Preliminary results show that shrimp RMR rose with increasing temperature as MMR declined, resulting in a decrease in aerobic scope (MMR - RMR) with increasing temperature. Aerobic scope declined to zero at ~38°C, corresponding to the observed CTM. In addition, aerobic scope at sublethal temperatures was significantly higher for the small size-class compared to the large size-class. Reductions in aerobic scope appear to be a good predictor of lethal stress and may explain differences in thermal tolerance between small and large size-classes.