Blue mussel (Mytilus edulis) produce byssal threads to anchor themselves to the substrate. These threads are always exposed to the surrounding environmental conditions. Understanding how environmental pH affects these threads is crucial in understanding how climate change can affect mussels. This work examines three factors (load at failure, thread extensibility, and total thread counts) that indicate the performance of byssal threads as well as condition index to assess impacts on the physiological condition of mussels held in artificial seawater acidified by the addition of CO2. There was no significant variation between the control (~786 μatm CO2 / ~7.98 pH/ ~2805 μmol kg-1 total alkalinity) and acidified (~2555 μatm CO2 / ~7.47 pH/ ~2650 μmol kg-1 total alkalinity) treatment groups in any of these factors. The results of this study suggest that ocean acidification by CO2 addition has no significant effect on the quality and performance of threads produced by M. edulis.