Of the many endoparasites found in racers, the lung fluke, _Neorenifer lateriporus_ was the only one identified and frequently observed in my study (Stewart, 1959). This is a digenetic trematode of the subfamily Reniferinae. The racer is its specific host. The life history is still unknown, but in other members of the subfamily, all of which parasitize snakes, an aquatic snail and a frog are required as hosts at different stages of the life cycle. Presumably _N. lateriporus_ has similar requirements. The two common local water snails, _Heliosoma trivolvis_ and _Physa anatina_, are both potential hosts. By far the most probable frog host is the leopard frog. In wet weather of July the recently metamorphosed leopard frogs leave the water and disperse to all habitats, probably carrying with them the parasites acquired in the tadpole stage. The racers in turn probably acquire their flukes by eating the young frogs in summer. In any case, the adult racers are nearly all parasitized, but the flukes have not been found in those racers that were less than one year old. During their first few months, the racers are too small to swallow leopard frogs, even the young. The flukes have been seen in the live racers mostly in May, when most adults are infested with the flukes. Seemingly at this season the flukes migrate forward into the mouth of the host. Probably this is the time when the flukes breed and lay eggs; if so, the eggs would pass through the digestive tract of the snake and escape with its feces. The latter are usually left in terrestrial situations unfavorable for the development of an aquatic stage, but perhaps some of the eggs are washed into ponds by heavy summer rains. In late summer and fall the flukes are not to be found in the mouths of the live snakes. Most complete records of the flukes present in racers were kept in 1959. The following table shows the numbers of racers examined and the percentage having flukes in that year. Ortenburger (1928:182) recorded lung flukes (_Renifer ellipticus_) from blue racers. In Maryland, McCauley (1945:76) also recorded numerous lung flukes (_Pseudorenifer_ sp.) in an immature racer 490 millimeters in total length. Parker (1941:34) recorded _Neorenifer septicus_ from racers collected at Reelfoot Lake, Tennessee, Greensboro, Georgia, and Kissimmee, Florida; also he recorded _N. georgianum_ from racers collected at Reelfoot Lake. _N. septicus_ was recorded by the same author from the water moccasin (_Agkistrodon piscivorus_) and _N. georgianum_ was also recorded from the king snake (_Lampropeltis getulus_).