Leptorhynchoides thecatus, an acanthocephalan of centrarchid fishes, occurs in the pyloric ceca and the anterior intestine of green sunfish, Lepomis cyanellus. One wk post-laboratory infection, L. thecatus is found in the pyloric ceca and the anterior portion of the intestine of green sunfish, but by 3 wk postinfection, worms are present only in the pyloric ceca. This study investigated whether differential mortality (higher mortality in the intestine than in the ceca) or anteriad emigration is responsible for localization in the ceca. Chi-square analysis was performed on data obtained from 149 1-worm laboratory infections. No significant difference was found in the number of worms recovered during the course of the study; therefore, differential mortality as a mechanism for worm localization is unlikely. Furthermore, there was a decrease in prevalence of intestinal worms with a concurrent increase in prevalence of cecal worms. It is concluded that L. thecatus is localized in the pyloric ceca of green sunfish due to anteriad emigration. This emigration appears to be complete by day 2 post infection.