A microsporidian parasite infecting the pheasantshell mussel, Actinonaiais pectorosa, was discovered in a freshwater system in the USA. The original description of this species placed it into the holding genus “Microsporidium”; however, the availability of ecological, environmental, histological, electron microscopy, and genetic data, suffice to provide a complete formal taxonomic account of this species. In this note, the genus Knowlespora n. gen. is erected and described to hold the type species: Knowlespora clinchi gen. et comb. nov., originally described as ‘Microsporidium clinchi’. A discussion is presented to suggest that this novel species is unlikely to be closely related to other molluscan microsporidians (all of which lack genetic data, to date) and highlights the diversity of oocyte-infecting species across the Microsporidia.