Ocean acidification poses an ongoing threat to marine organisms, and early life stages are believed to be particularly sensitive. The boreal calanoid copepod Calanus finmarchicus seasonally dominates the standing stock of zooplankton in the northern North Sea and North Atlantic, and due to its size and abundance is considered an ecological key species linking energy from primary producers to higher trophic levels. To examine whether the early stages of C. finmarchicus are particularly vulnerable to elevated levels of CO₂, eggs and nauplii were subjected to different levels of CO₂-acidified seawater for 1 wk. The first experiment, with eggs as the starting point, revealed no marked effect on hatching success, but a significant reduction in nauplii survival during incubation at 8800 ppm CO₂. In addition, a significant decrease in ontogenetic development rate during incubation at 8800 ppm CO₂ was observed in this experiment. In the second experiment, where third-stage nauplii represented the starting point, no significant effects on ontogenetic development and survival following exposure to pCO₂ ≥ 7700 ppm were observed. Data suggest that the two first nauplii stages, which are fed endogenously, may be more vulnerable and therefore likely to represent the “bottleneck” for this species in a more acidic ocean. However, the absence of significant effects in the most sensitive stages during exposure to 2800 ppm CO₂, a level that is well above worst-case scenario predictions for year 2300 (approximately 2000 ppm CO₂), suggests that this species may be generally robust to direct effects of ocean acidification.