Infection of a threespine stickleback (Gasterosteus aculeatus) by a specialist tapeworm parasite (Schistocephalus solidus) is known to reduce host fitness by reducing their fecundity and increasing the likelihood of predation through behavioral manipulation. Some populations of stickleback respond to tapeworm infection by developing peritoneal fibrosis, which encases the invading parasite in fibrous connective tissue. Fibrosis can reduce the growth of or even kill the parasite; however, this response may have its own set of fitness consequences. Fibrosis is known to reduce the fecundity of females and the nesting success of males, and it may have additional locomotory costs. The presence of fibrosed tissue may affect the stiffness and elastic efficiency of the body and could therefore impair locomotion. In order to examine the potential biomechanical costs of the peritoneal fibrosis immune response we induce fibrosis in sticklebacks through intraperitoneal injection of an immune adjuvant. We then use high speed videography to record C-start escape responses in fish with and without peritoneal fibrosis. We euthanized the fish and estimated the flexural stiffness of their body using a three-point bending test. Finally, we dissected the fish and visually scored them for the severity of their fibrosis response. We use the connection between fibrosis levels and body stiffness to explore variation in escape response kinematics. This is one of the first studies of the biomechanical cost of an immune response and could have implications on our understanding of the evolution of pathogen-host response in early vertebrates.