Microsporidian parasites are ubiquitous in aquatic ecosystems and infect a broad range of animal phyla.  We used molecular techniques to determine the phylogenetic placement and classification of a microsporidian (Microsporidia sp.) that has recently been studied in populations of the amphipod Gammarus pseudolimnaeus in southwest Michigan.  Microsporidia sp. primarily infects female Gammarus and can alter host fitness as well as population sex ratios.  We used DNA sequence data from the microsporidian 16s rRNA gene to place Microsporidia sp. among other aquatic microsporidia taxa and evaluate whether it should be considered a unique species.  PCR analysis was used to determine if vertical transmission from infected mothers to their offspring was the typical route of infection.  Our results suggest that this microsporidian belongs in the genus Dictyocoela and should be considered a unique species.  Dictyocoela contains at least six other microsporidian species, all of which infect European amphipod hosts.  The results are discussed in the context of the evolution of disease transmission and how parasites are defined.