The impact of medium-term exposure to CO₂-acidified seawater on survival, growth and development was investigated in the North Atlantic copepod Calanus finmarchicus. Using a custom developed experimental system, fertilized eggs and subsequent development stages were exposed to normal seawater (390 ppm CO₂) or one of three different levels of CO₂-induced acidification (3300, 7300, 9700 ppm CO₂). Following the 28-day exposure period, survival was found to be unaffected by exposure to 3300 ppm CO₂, but significantly reduced at 7300 and 9700 ppm CO₂. Also, the proportion of copepodite stages IV to VI observed in the different treatments was significantly affected in a manner that may indicate a CO₂-induced retardation of the rate of ontogenetic development. Morphometric analysis revealed a significant increase in size (prosome length) and lipid storage volume in stage IV copepodites exposed to 3300 ppm CO₂ and reduced size in stage III copepodites exposed to 7300 ppm CO₂. Together, the findings indicate that a pCO₂ level ≤2000 ppm (the highest CO₂ level expected by the year 2300) will probably not directly affect survival in C. finmarchicus. Longer term experiments at more moderate CO₂ levels are, however, necessary before the possibility that growth and development may be affected below 2000 ppm CO₂ can be ruled out.