ABSTRACT: Marine calcifiers, especially those in larval and juvenile stages, are thought to be most vulnerable to ocean acidification (OA) due to the effects of carbon dioxide (CO₂) on growth and calcification. However, recent evidence in lobsters is contradictory. We monitored molting activity, length, and weight in early benthic phase Homarus americanus (Milne-Edwards 1837) over 90 to 120 d under 3 targeted CO₂ partial pressures (pCO₂; 400, 1000, and 2000 µatm) to determine how elevated CO₂ affects growth at this life stage. Lobsters exposed to higher pCO₂ over that 90 to 120 d period exhibited altered intermolt period length and decreased growth increments (length and weight). Lobsters in the elevated CO₂ treatments were also more susceptible to shell disease. These results suggest juvenile lobsters may remain smaller, and thus more susceptible to predation, for a longer period of time and may be more susceptible to disease in a high CO₂ ocean.