Diplostomum is a taxonomically problematic genus of trematodes, with many members harmful at the metacercarial stage to fish in aquaculture. We found metacercariae in the eye vitreous humor of the rudd, Scardinius erythrophthalmus; the bleak, Alburnus alburnus; and the roach, Rutilus rutilus, in Karelia (Russia) and identified them as Diplostomum numericum based on a combination of morphological and ecological data. Previously, D. numericum was thought to be conspecific with Diplostomum gavium. However, our phylogenetic analysis based on the sequences of the cox1 mitochondrial DNA gene did not support this hypothesis. We demonstrated the conspecificity of D. numericum with the larval trematode Diplostomum sp. Lineage 3 of Blasco-Costa et al. (2014) from the eye vitreous humor of European salmonid and cyprinid fishes. Therefore, we resurrected the species D. numericum and identified Diplostomum sp. Lineage 3 of Blasco-Costa et al. (2014) as D. numericum. Finally, we hypothesized that D. numericum was, in fact, conspecific with Diplostomum colymbi and that the latter was distinct from D. gavium sensu stricto.