Shellfish production is often affected by bacterial pathogens that cause high losses in hatcheries and nurseries. We evaluated the relative survival of larvae and juveniles of 3 Crassostrea virginica oyster lines: (1) GHP, a Rhode Island line; (2) NEHY, a line resistant to dermo and multinucleated sphere X diseases; and (3) FLOWERS, a line resistant to Roseovarius oyster disease, experimental challenge with Vibrio spp. isolates RE22 and RE101, causative agents of bacillary necrosis in Pacific oyster larvae, and the type strain of Roseovarius crassostreae, causative agent of Roseovarius oyster disease. All of the isolates were able to induce significant mortalities in oyster larvae and juveniles. Susceptibility to bacterial challenge in larvae was significantly higher at 25°C than at 20°C. Susceptibility decreased with oyster age; mean survival time ranged from 24 h in oyster larvae to more than 6 wk in juveniles. Significant differences in susceptibility to bacterial challenge were observed between oyster lines; NEHY was the most resistant line overall. Extracellular products (ECPs) from Vibrio sp. RE22 and R. crassostreae, as well as viable bacteria, were toxic to hemocytes from the 3 oyster lines, suggesting that ECPs are involved in pathogenesis and that external and mucosal barriers to infection are major contributors to resistance to bacterial challenge. These protocols will be useful in the elucidation of mechanisms of bacterial pathogenesis and resistance to infection in oysters.