AbstractVertebrate hosts commonly harbour concurrent infections of different helminth species which may interact with each other in a synergistic, antagonistic or negligible manner. Direct interactions between helminths that share a common site in the host have been regularly reported, but indirect interactions between species that occur in different sites are rarely described, especially in fish hosts. Plerocercoids ofLigula intestinalisare common infections of the peritoneal (body) cavity of roach (Rutilus rutilus) in freshwater habitats. These larval cestodes can cause extensive systemic pathologies to the fish host, which in turn may alter its susceptibility as a target host for other helminth species. The present study, using an existing dataset, investigates the influence ofL. intestinalis(ligulosis) on frequently occurring eyefluke infections in roach sampled from a lake in south-east England. The occurrence of two species of eyefluke (Diplostomumsp. andTylodelphyssp.) in the roach population demonstrated no significant levels of interaction with each other. The prevalence but not mean intensity or abundance ofDiplostomumsp. was significantly increased in ligulosed roach, while the incidence ofTylodelphyssp. remained unchanged. Analyses of bilateral asymmetry in the occurrence of eyeflukes in left and right eyes of infected fish demonstrate thatTylodelphyssp. shows significant asymmetry in non-ligulosed roach, which is not replicated in ligulosed individuals. In contrast,Diplostomumsp. shows no evidence of asymmetry in either ligulosed or non-ligulosed fish.