<p>Estuarine organisms, such as juvenile penaeid shrimp, experience fluctuating oxygen pressures on a daily basis. In coastal waters of the southeastern United States, severe hypoxia (&lt; 4&ndash;6 kPa) is common in the summer, also a time during which bacterial concentrations in seawater are high. In response to invading bacteria, crustaceans mount an immune defense resulting in aggregations of circulating hemocytes. These aggregates can be trapped in and obstruct hemolymph flow through the gills, inhibiting oxygen uptake. Hypoxia itself is also known to inhibit immune function. In this study we investigated some key characteristics of penaeid shrimp that are likely to be associated with their ability to cope with hypoxia and bacterial infection. We compare critical Po₂, the oxygen pressure below which oxygen uptake depends on available ambient oxygen, in two commercially important shrimp species: wild <i> Litopenaeus setiferus</i>, the Atlantic white shrimp, and aquacultured <i> Litopenaeus vannamei</i>, the Pacific whiteleg shrimp. We also compare whole body lactate concentrations following acute exposure to severe environmental hypoxia (5.3 kPa), and injection with a sub-lethal dose of bacteria (<i> Vibrio campbellii</i> 90-69B3), in aquacultured <i>L. vannamei</i> and in wild-caught <i>Farfantepenaeus duorarum</i>, the Atlantic pink shrimp. LD₅₀ tests indicate that the virulence of <i> V. campbellii</i> in <i>L. setiferus</i> (LD₅₀ = 6.4 &times; 10⁵ CFU g^&minus;1 shrimp) is similar to that previously determined in <i>L. vannamei</i> (LD₅₀ = 3.06 &times; 10⁵ CFU g^&minus;1 shrimp). We found no difference between <i>L. vannamei</i> and <i>L. setiferus </i> in critical Po₂, which fell between 3.5&ndash;5.2 kPa in both species. Whole body lactate concentration was measured in shrimp held in normoxia (>16 kPa) or hypoxia and injected with saline or bacteria. There were no effects of exposure in <i>L. vannamei</i>, however lactate concentration in <i>F. duorarum</i> increased by 60% in shrimp exposed concurrently to hypoxia and injected-bacteria, compared to saline-injected shrimp exposed to hypoxia and <i>Vibrio</i>-injected shrimp exposed to normoxia. This is consistent with previous findings in our lab, that hemocyanin in <i>L. vannamei</i> has a higher concentration and O₂ affinity, resulting in better tissue oxygenation, than that in wild species of Atlantic shrimp. These data suggest that aquacultured <i>L. vannamei </i> has an adaptive advantage over at least one species of wild penaeid shrimp in coping with hypoxia and bacterial infection. </p>