Parasites exert substantial costs on their hosts. Thus, natural selection should favour behavioural defenses that reduce hosts’ exposure to parasites. This prediction has rarely been tested for aquatic hosts exposed to parasites. I designed experiments to test if fathead minnows could detect cercariae of the trematode, Ornithodiplostomum sp. and engage in antiparasite behaviours to avoid them. Minnows exposed to cercariae formed 20.1% tighter shoals compared to water controls. Further, minnows greatly reduced their overall activity, but only when they were exposed for a second time. The latter result is important because it provides the first indication that hosts can learn to avoid parasites. Lastly, I tested if epidermal club cells play a defensive role against cercariae. Club cells did not, but other components of the epidermis, probably mucus cells, decreased cercarial infectivity by 61-68%. My results show that fish can detect, learn, and ultimately avoid aquatic larval stages of parasites. / 105 leaves : ill. ; 29 cm.