The association between the hermit crab Pagurus longicarpus Say and an ectosymbiotic colonial hydroid of the genus Hydractinia was studied to determine whether the presence of Hydractinia affects the resistance of Pagurus longicarpus to predation. Observations of tethered hermit crabs confirmed the importance of predation by the blue crab, Callinectes sapidus Rathbun, on the Pagurus longicarpus population in Nantucket Harbor, Massachusetts. In a predation experiment, caged hermit crabs in Littorina littorea (L.) shells with and without Hydractinia were exposed to single blue crabs. Shell damage indicated that blue crabs attacked hermit crabs in both shell types in roughly equal proportions: 65% of hermit crabs in shells with Hydractinia were attacked compared to 69% of hermit crabs in shells without Hydractinia. Only 20% of hermit crabs in shells with the hydroid survived attack, however, compared to 54% in shells without Hydractinia. Continuous observation of predation in aquaria showed that hermit crabs in shells with Hydractinia had significantly shorter survival times when attacked by a blue crab than hermit crabs in shells without the colonial hydroid. The crushing resistance of shells was measured with an Instron™ testing machine. A comparison of the regression equations of shell mass (g) vs. crushing resistance (N) between shell types showed that shells with Hydractinia had a lower slope than shells without Hydractinia, although because of a wide range in crushing resistance, this difference was not significant. Shells with Polydoran worm burrows, however, were significantly weaker than shells without burrows, and shells with Hydractinia were more likely to hold worms than shells without Hydractinia. The positive correlation between Hydractinia and shell-degrading worms explains the vulnerability of hermit crabs in shells with the colonial hydroid to predation by blue crabs. Other studies have shown that hermit crabs may benefit from having Hydractinia on their shells because the hydroid prevents large and potentially harmful ectosymbionts, such as Crepidula species, from colonizing. Thus, for Pagurus longicarpus the costs and benefits of the association with Hydractinia may change depending upon the presence of other organisms and this may shift the nature of the association with Hydractinia from mutualism to commensalism to parasitism