The objectives of this study were twofold: to determine the sensitivity of gill biopsies used for the screening of Pseudodactylogyrus bini in subclinically infested American eels (Anguilla rostrata) and to evaluate the effect of bath anesthesia on the proportion of infested eels. Fifty-four eels were euthanized following either intracoelomic injection of T61 or deep anesthesia with tricaine methanesulfonate. Proportions of eels infested by P. bini diagnosed by gill biopsies taken through the gill slit and by microscopic examination of the entire gill arches were compared. The euthanasia protocol had no significant effect on parasite abundance. The prevalence of monogeneans detected by examination of nonterminal antemortem gill biopsies and by complete gill examinations was 20% and 70%, respectively. The sensitivity of gill biopsies (29%) to detect P. bini in subclinically infested eels is therefore low, and should be used for group screening rather than individual screening.