Shell growth, survival, and physiology were compared between diploid Suminoe (Crassostrea ariakensis) and eastern oysters (Crassostrea virginica) under conditions simulating a U.S. subtropical estuary. Two age groups (4 mo and 28 mo) of both oyster species were grown for 9 mo (December 2006 through August 2007) in quarantine mesocosms (700 L) supplied with ambient flowing (≥ 10 L/min) water (annual temperature range, 18.6–30.4°C; salinity, 28–37.7). There was no difference in overall rates of shell growth between the 2 oyster species over the 9-mo period. Specific growth rates for C. ariakensis did not differ over time, but they did for C. virginica. The growth rate of C. virginica was slowest in the winter (8.9 × 10^-4 mm²/day) and fastest in the spring (43.5 × 10^-4 mm²/day). Mortality of both species rose abruptly in April 2007, and all (100%) remaining C. ariakensis were dead by the end of the study. Although 28% of the remaining C. virginica died in April 2007, there was little further mortality in this species before the study was terminated in August 2007. Physiological responses of both species of oysters were compared under seasonal temperate euhaline quarantine conditions to understand better how temperature affects these species without the confounding unexplained mortality encountered in the subtropical mesocosms. The clearance rate of C. ariakensis (1.2 L/g/h) was half that of C. virginica (2.2 L/g/h) during the summer (25°C); however, respiration rates for C. ariakensis (2.6 ml O₂/g/h) and C. virginica (2.5 ml O₂ g/h) were similar. The low clearance rate of C. ariakensis during the summer resulted in a negative scope for growth (-36.2 J/g/h) during this season. During the winter, C. ariakensis remained physiologically active when water temperatures were as low as 2°C; C. virginica were quiescent during this time. We conclude that the “Oregon” strain of C. ariakensis tested will not thrive in the warm subtropical waters of the U.S. southeastern coast; however, given its native range in Asia, we do not discount the possibility of an adaptation to warmer temperatures over time.