Ocean acidification can affect the immune responses of fish, but effects on pathogen susceptibility remain uncertain. Pacific herring Clupea pallasii were reared from hatch under 3 CO₂ partial pressure ( pCO₂) treatments (ambient, ∼650 µatm; intermediate, ∼1500 µatm; high, ∼3000 µatm) through metamorphosis (98 d) to evaluate the effects of ocean acidification on bioenergetics and susceptibility to an endemic viral disease. Mortality from viral hemorrhagic septicemia (VHS) was comparable between herring reared under ambient and intermediate pCO₂ (all vulnerability testing at ambient pCO₂). By contrast, fish reared under high pCO₂ experienced significantly higher rates of VHS mortality, and the condition factor of survivors was significantly lower than in the other pCO₂ treatments. However, the prevalence of infection among survivors was not influenced by pCO₂ treatment. Pre-flexion larval development was not affected by elevated pCO₂, as growth rate, energy use, and feeding activity were comparable across treatments. Similarly, long-term growth (14 wk) was not affected by chronic exposure to elevated pCO₂. Herring reared under both elevated pCO₂ treatments showed an average reduction in swimming speed; however, wide intra-treatment variability rendered the effect nonsignificant. This study demonstrates that the VHS susceptibility and bioenergetics of larval and post-metamorphic Pacific herring are not affected by near-future ocean acidification predicted for coastal systems of the North Pacific. However, increased susceptibility to VHS in fish reared under 3000 µatm pCO₂ indicates potential health and fitness consequences from extreme acidification.