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.