Freshwater mussels may be exposed to elevations in mean partial pressure of carbon dioxide (pCO 2 ) caused by both natural and anthropogenic factors. The goal of the present study was to assess the effects of a 28-d elevation in pCO 2 at 15 000 and 50 000 μatm on processes associated with biomineralization, ion regulation, and cellular stress in adult Lampsilis siliquoidea (Barnes, 1823). In addition, the capacity for mussels to compensate for acid-base disturbances experienced after exposure to elevated pCO 2 was assessed over a 14-d recovery period. Overall, exposure to 50 000 μatm pCO 2 had more pronounced physiological consequences compared with 15 000 μatm pCO 2 . Over the first 7 d of exposure to 50 000 μatm pCO 2 , the mRNA abundance of chitin synthase (cs), calmodulin (cam), and calmodulin-like protein (calp) were significantly affected, suggesting that shell formation and integrity may be altered during pCO 2 exposure. After the removal of the pCO 2 treatment, mussels may compensate for the acid-base and ion disturbances experienced during pCO 2 exposure, and transcript levels of some regulators of biomineralization (carbonic anhydrase [ca], cs, cam, calp) as well as ion regulation (na + -k + -ATPase [nka]) were modulated. Effects of elevated pCO 2 on heat shock protein 70 (hsp70) were limited in the present study. Overall, adult L. siliquoidea appeared to regulate factors associated with the control of biomineralization and ion regulation during and/or after the removal of pCO 2 exposure. Environ Toxicol Chem 2018;37:538–550.