ABSTRACT: The attachment strength of sessile intertidal organisms is continuously challenged by the hydrodynamic forces generated by breaking waves. This study explores mechanisms by which the attachment strength, or tenacity, can vary for one of the dominantcompetitors for space in this environment, the marine mussel. Tenacity was measured for 2 co-existing mussel species, Mytilus californianus and Mytilus trossulus, either solitary or within a bed (= bed mussels). The tenacity of M.californianus was higher than M. trossulus, due to increased byssal thread thickness, and the tenacity of solitary mussels was higher than bed mussels, due to the presence of more byssal threads per mussel. These tenacity measurements werecoupled with modeled hydrodynamic forces to predict the probability of dislodgment due to wave action. For a given water velocity, the predicted probability of dislodgment of M. californianus was lower than that of M. trossulus because thelatter produces relatively thinner threads (reducing tenacity) and a relatively more voluminous shell (increasing hydrodynamic loading). Compared to solitary mussels, bed mussels had a lower probability of dislodgment for a given water velocity (despitetheir lower tenacity) because they are subjected to relatively smaller hydrodynamic forces. These predictions are consistent with field observations that mussels typically form dense aggregations and that M. trossulus rarely inhabits highlywave-exposed shores.