Abstract: The combined effect of increased ocean warming and elevated carbon dioxide in seawater is expected to have significant physiological and ecological consequences at many organizational levels of the marine ecosystem. In the present study, juvenile mussels Mytilus chilensis were reared for 80 din a factorial combination of two temperatures (12 and 16 degrees C) and three pCO(2) levels (380, 700, and 1000 μatm). We investigated the combined effects of increasing seawater temperature and pCO(2) on the physiological performance (i.e. feeding, metabolism, and growth). Lower clearance rate (CR) occurred at the highest pCO(2) concentration (1000 μatm) compared with the control (380 μatm) and with the intermediate concentration of pCO(2) (700 μatm). Conversely, CR was significantly higher at 16 degrees C than at 12 degrees C. Significant lower values of oxygen uptake were observed in mussels exposed to 1000 μatm pCO(2) level compared with those exposed to 380 μatm pCO(2). Scope for growth (SFG) was significantly lower at the highest pCO(2) concentration compared with the control. Mussels exposed to 700 μatm pCO(2) did not show significantly different SFG from the other two pCO(2) treatments. SFG was significantly higher at 16 degrees C than at 12 degrees C. This might be explained because the experimental mussels were exposed to temperatures experienced in their natural environment, which are within the range of thermal tolerance of the species. Our results suggest that the temperature rise within the natural range experienced by M. chilensis generates a positive effect on the processes related with energy gain (i.e. feeding and absorption) to be allocated to growth. In turn, the increase in the pCO(2) level of 1000 μatm, independent of temperature, adversely affects this species, with significantly reduced energy allocated to growth (SFG) compared with the control treatment.