Laboratory-reared cystacanths of Leptorhynchoides thecatus (Acanthocephala: Rhadinorhynchidae) were used to study the effect of cystacanth size on adult success and the factors that influence cystacanth size within the intermediate host. To assess how host size and intensity of infection influence cystacanth size, infected amphipods (Hyalella azteca) were measured, and sex, length, and width of cystacanths were determined. After a subset of cystacanths was measured, small- and large-size classes of cystacanths were designated. To determine how cystacanth size relates to adult size, green sunfish (Lepomis cyanellus) were fed 10 large or small cystacanths. Fish were dissected 6 wk after infection, and worms were removed. After adult worms were permanently mounted on slides, their length and width were measured. Intensity of infection and amphipod size significantly influenced cystacanth size in that large amphipods harbored larger cystacanths than did small amphipods and heavy infections produced smaller cystacanths than did light infections. Adult worms from the small and large cystacanth-size classes showed no significant difference in size; however, large cystacanths had a significantly higher establishment and survival than did small cystacanths: 40% of large worms and 14% of small worms were recovered. The results of this study indicate that host size and host sharing influence cystacanth size and that cystacanth size is an important factor in determining adult success.