A growing body of research on calcifying marine invertebrates suggests that ocean acidification will have deleterious effects on development and various physiological processes in these organisms. In laboratory experiments designed to mimic seawater chemistry in future oceans, we examined the effect of pH reduction, driven by the carbon dioxide (CO₂) acidification of seawater, on larvae of the red abalone, Haliotis rufescens. Following development under CO₂-acidified conditions, we measured 2 indicators of physiological response to low pH in 4 stages of larval development: (1) tolerance of acute thermal challenges and (2) quantitative real-time polymerase chain reaction-determined expression of 2 genes involved in shell formation (engrailed and ap24). The results showed that low pH (pH 7.87 vs. pH 8.05 for control treatments) had a significant effect and decreased larval thermal tolerance for some developmental stages (pretorsion and late veliger), but not for others (posttorsion and premetamorphic veligers). In contrast to the thermal tolerance data, decreased pH did not affect the expression pattern of the 2 shell formation genes in any of the abalone larval stages. The results indicate larval stages were differentially sensitive to low pH conditions and this variability may play into the resilience of individual species to withstand environmental change in the longer term.