In the summer of 2010, the Damariscotta River Estuary, ME, experienced the first large multinucleate sphere X disease (Haplosporidium nelsoni) outbreak in commercial stocks of the eastern oyster (Crassostrea virginica). In 2012, biofouling organisms were sampled in and around the commercial oyster operations with the intent of looking for the presence of the parasite to see which, if any, biofouling species may be acting as a reservoir host and harboring the parasite. Organisms collected included tunicates, gastropods, polychaetes, and arthropods, as well as plankton samples from the surrounding water column. A quantitative polymerase chain reaction assay for H. nelsoni using a TaqMan minor groove binder probe, which was originally developed for detection of the parasiteDNAin oysters, was applied to biofouling species. The protozoan parasite H. nelsoniDNA was present in up to 70% of tunicate samples and about 30% of plankton samples. The highest parasite DNA copies (averaging 4 × 10⁴ copies) were detected in tunicates identified as Styela sp., which have been recognized as invasive species in many coastal areas. This study provides evidence for the presence of potential reservoir species that occur close to commercial oyster operations. Reduction or removal of these species may reduce the infectious pressure of this parasite on commercial operations.