As intertidal communities face predicted increases in temperature and hydrodynamic disturbances due to climate change, organisms may find it necessary to reallocate energy among normal physiological processes for survival. Specifically, mussels must distribute energy among processes such as attachment, shell growth, metabolism, and reproduction, all of which may be influenced by seasonality and other natural environmental fluctuations (temperature, pCO2, etc.). Examining energy distributions is the first step in predicting responses to environmental change. In the present study, we investigated the energetic cost of byssus production for the intertidal mussel Mytilus trossulus. After performing an initial pilot study to establish methods for manipulating byssus production by controlling how often the mussels had to replenish byssal threads (ie. daily, weekly, never), we exposed collected mussels to each treatment in triplicate for a four-week period. We then assessed the energetic cost associated with byssus production in relation to energy allocated to other processes such as growth and development. For all measures (length, width, height, shell mass, etc.), forcing the mussel to produce more byssus resulted in a decrease in growth rate; this was significant for length and shell mass. For this study, activities for making longer shells were first to be forfeited as mussels were manipulated into manufacturing greater amounts of byssus during a season where developing reproductive structures were of highest priority. This research identifies byssus production as a major energetic constraint in mussels, which play an important role both economically through aquaculture as well as in intertidal marine ecosystems. As these globally marketed shellfish are placing more amounts of energy towards byssus production, less energy is available for growth. Coupled with modeling predictions, this information could be useful for aquaculture practices as well as understanding physical changes mussels undergo in response predicted climate change.