The objectives of this study were (1) to use egg development in the adult worm of Proterometra macrostoma as a means to delineate new infections of centrarchid fishes with this digenean and (2) to assess histopathology associated with infection of the fish host. An equal mix of adult worm ages/types based on eggs stages was produced by exposing individual hatchery-reared bluegill to the same number of cercariae at weekly intervals. These results confirmed the use of egg stages for differentiating recent vs. older infections in fishes from North Elkhorn Creek, Scott County, Kentucky, during June and July 2002. Between 50.5% and 74.4% of worms recovered from naturally infected warmouth, bluegill, and longear sunfish lacked eggs or possessed eggs in early cleavage with a large vitelline mass implicating summer as a period for new infections of fishes with this worm. A proportionately smaller number of P. macrostoma containing eggs in late cleavage (11.4–20.5%) or with miracidia (14.6–29.7%) was also found at this time. In experimental infections of hatchery-reared bluegill, damage to the host mucosa appeared restricted to the attachment site. The oral sucker of adult worms constricted the mucosa of the host esophagus and stomach, causing hemorrhaging and epithelial necrosis. Complete detachment of host tissue in the oral sucker of many worms was confirmed through serial sections, yet no experimentally infected fish died before they were sacrificed at the end of this study.