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Cercarial behavior patterns were examined in four species of frog lung flukes (Haematoloechus spp.). Cercariae of Haematoloechus complexus, Haematoloechus medioplexus, Haematoloechus longiplexus, and Haematoloechus varioplexus were exposed to three species of experimental arthropods and an inanimate control. The number of cercariae attached to an experimental host at 5 min postexposure was recorded. Haematoloechus longiplexus and H. complexus cercariae attached to experimental hosts at higher rates than cercariae of H. medioplexus and H. varioplexus. Cercariae of H. longiplexus attached to experimental hosts in approximately the same numbers as H. complexus, but H. longiplexus penetrated only damselfly naiads, and only at the base of the zygopteran caudal gills. Cercariae of H. complexus, a second intermediate host generalist, were able to penetrate and enter several arthropod species at the intersegmental membranes. Haematoloechus medioplexus and H. varioplexus are restricted to development in dragonfly naiads and cercariae rarely attached to and never penetrated experimental hosts. These behavioral patterns dictate the range of hosts suitable for metacercarial development of H. complexus, H. longiplexus, H. medioplexus, and H. varioplexus. The evolution of disparate patterns of behavior among the cercariae of these four congeners has directly affected subsequent patterns of transmission to the definitive host.