Among the photosynthetically active dinophytes, the Kryptoperidiniaceae are unique in having a diatom as endosymbiont instead of the widely present peridinin chloroplast. Phylogenetically, it is unresolved at present how the endosymbionts are inherited, and the taxonomic identities of two iconic dinophyte names,Kryptoperidinium foliaceumandKryptoperidinium triquetrum, are also unclear. Multiple strains were newly established from the type locality in the German Baltic Sea off Wismar and inspected using microscopy as well as molecular sequence diagnostics of both host and endosymbiont. All strains were bi-nucleate, shared the same plate formula (i.e., po, X, 4′, 2a, 7′′, 5c, 7s, 5′′′, 2′′′′) and exhibited a narrow and characteristically L-shaped precingular plate 7′′. Within the molecular phylogeny of Bacillariaceae, endosymbionts were scattered over the tree in a highly polyphyletic pattern, even if they were gained from different strains of a single species, namelyK. triquetrum. Notably, endosymbionts from the Baltic Sea show molecular sequences distinct from the Atlantic and the Mediterranean Sea, which is the first report of such a spatial fragmentation in a planktonic species of dinophytes. The two namesK. foliaceumandK. triquetrumare taxonomically clarified by epitypification, withK. triquetrumhaving priority over its synonymK. foliaceum. Our study underlines the need of stable taxonomy for central questions in evolutionary biology.