Large freshwater mussels (Unionida) are long-lived, have large bodies, and produce thousands to millions of larvae (glochidia) that usually must attach to host fish tissue to complete their life cycle. This is an obligate parasitic stage of mussel larvae. However, less than one in onemillion find a suitable host and survive. The degree of host specificity varies among unionid species, from specialists that can successfully parasitize only one or a few closely related fish species to generalists that can complete development on a taxonomically broad range of fish species. In addition, freshwater mussels are among the most threatened groups of animals. This is due to habitat destruction, the introduction of non-native species, and the loss of host fish on which their larvae (glochidia) are obligate parasites. Glochidiosis harms fish by affecting their growth; on the other hand, freshwater mussels play an important role in freshwaters by improving water quality and ridding the water of bacteria, algae, and pollutants, they are an indicator species of water quality. During our parasitological survey of fish from the Macedonian part of Lake Prespa in April 2022, many glochidia were found on the gills, skin, and fins of two endemic fishes, Prespa roach (Rutilus prespensis) and Prespa nase (Chondrostoma prespense), in the range of tens to thousands on one host. We thus recorded these two endemic species as new hosts of A. cygnea.