Metacercariae survival patterns and their distribution in second intermediate odonate hosts were examined for 4 species of frog lung flukes. Surveys of aquatic larvae and recently emerged teneral dragonflies and damselflies indicated that prevalence and mean abundance of Haematoloechus spp. metacercariae were significantly lower in teneral dragonflies than larval dragonflies, while there was no significant difference in prevalence or mean abundance of Haematoloechus spp. metacercariae among larval and teneral damselflies. Experimental infections of dragonflies indicated that metacercariae of Haematoloechus coloradensis and Haematoloechus complexus were located in the head, thorax, and branchial basket of dragonflies, whereas metacercariae of Haematoloechus longiplexus and Haematoloechus parviplexus were restricted to the branchial basket of these hosts. Metacercariae of H. coloradensis, H. complexus, and H. longiplexus infected the head, thorax, and abdomen of damselflies, but these insects were resistant to infection with H. parviplexus. Subsequent metamorphosis experiments on experimentally infected dragonflies indicated that most metacercariae of H. longiplexus were lost from the branchial basket during metamorphosis, but most metacercariae of H. coloradensis, H. complexus, and H. parviplexus survived dragonfly metamorphosis. These observations suggest that the observed ecological host specificity of H. longiplexus in semiterrestrial leopard frogs may be due to few metacercariae of H. longiplexus reaching these frogs in a terrestrial environment. Because of the uncertain validity of Haematoloechus varioplexus as a distinct species from its synonym H. parviplexus, their morphological characters were reevaluated. The morphological data on H. varioplexus and H. parviplexus indicate that they differ in their acetabulum length and width, ovary shape, testes length, and egg length and width. Experimental infections of plains leopard frogs, northern leopard frogs, and bullfrogs with worms from bullfrogs indicate that the synonymy of H. parviplexus with H. varioplexus is not warranted, and that these flukes are distinct species, i.e., H. parviplexus in bullfrogs and H. varioplexus in plains leopard frogs and northern leopard frogs.