The morphology, ultrastructure, genetics, and morphometrics of a species of Diplostomum von Nordmann 1832 (Digenea: Diplostomidae), isolated from the European flounder (Platichthys flesus (L.)) caught off the northwest coast of Portugal, are characterized. The metacercarial stage was found unencysted in the lens capsule of the eye. Light microscopical observations revealed the existence of some variability in specimen shape and size, with two morphotypes, referred to as "round" and "long", being apparent. Scanning electron microscopy revealed a smooth, unarmed tegument, with the lappet region being the most irregular and porose. Both the oral and ventral suckers were provided with a series of papillae, which presented very distinctive ultrastructural features and were particularly conspicuous in the case of the ventral sucker. The two morphotypes detected were found to have 100% genetic correspondence in the 18S + ITS1 + 5.8S region of the rDNA. Since the genetic data for this metacercaria differed from those of the species of Diplostomum available in GenBank, a description of a new genotype (accession number GQ370809) is provided. The molecular phylogenetic analyses, in conjunction with principal components and cluster analyses based on morphometric data, revealed the existence of consistent differences between the Diplostomum sp. metacercariae from flounder compared with Diplostomum spathaceum, Diplostomum mergi, Diplostomum pseudospathaceum, and Diplostomum paracaudum. The latter of these species was found to be the most similar to the present material. Our results do not support an evolutionary separation of the European and North American species of Diplostomum.