Technical Abstract: Macroscopic sarcocysts are often observed in ducks, but at present their taxonomic status remains uncertain owing to the fact that experimental studies have determined that ducks serve as intermediate hosts for several such parasites in the genus Sarcocystis. One such species, Sarcocystis rileyi was long ago established to involve the shoveler duck (Anas clypeata) and the striped skunk (Mephitis mephitis) as its intermediate and definitive hosts. Here, we employed light microscopy, electron microscopy, and DNA sequencing to more precisely describe diagnostic attributes of parasites presumed to represent S. rileyi, occurring in a naturally-infected mallard duck (Anas platyrhynchos). By light and transmission electron microscopy, sarcocysts from the mallard duck resembled the S. rileyi described from A. clypeata. We document 18S, ITS-1, and 28S rDNA sequences from the mallard duck, the first for S. rileyi from any host. Sequences of conserved and variable portions of nuclear ribosomal DNA indicated that S. rileyi is related to, but distinct from, parasites employing opossums and their definitive host (including S. neurona and S. falcatula). Diagnostic ultrastructural features and nucleotide sequences should aid in future studies and communications regarding this parasitic taxon, which lends itself to experimentation because its sarcocysts are macroscopic and easily excised from infected ducks.