The small nemertean <i>Maculaura alaskensis</i> is used as a model for studies of pilidiophoran development. During several microinjection experiments, it became clear several batches of oocytes obtained from wild-caught females contained an intracellular pathogen. Infected oocytes have large vesicles containing dozens to hundreds of refractile oval objects. Examination of oocytes with DIC and confocal microscopy showed the spores within the vesicles were diplokaryotic and contained a coiled tube, traits that are diagnostic of the phylum Microsporidia. The Microsporidia are a group of obligate intracellular parasites that infect cells of protists and animals. No other microsporidian has ever been found infecting cells of nemerteans and the association between <i>M. alaskensis</i> and this microsporidian is, thus far, undocumented. For my thesis, I described morphological characteristics, molecular phylogeny, and geographic range of the microsporidian. Additionally, I observed parasitic influences on development of infected <i>M. alaskensis</i> and explored other potential host species. / 2021-04-30