Parvatrema borealis was described from sporocysts and cercariae found in Gemma gemma taken near Boothbay Harbor, Maine. Metacercariae, similar in morphology and presumably conspecific with the cercariae were found encysted in G. gemma Nereis limbata and Nereis virens occur together with G. gemma on the coast of Martha''s Vineyard, Massachusetts. These polychaetes are often, though lightly, infected with metacercariae that are indistinguishable morphologically from those found in G. gemma from Maine. Exposure of small N. virens to cercariae from G. gemma taken at Martha''s Vineyard resulted in heavy infection. These meta-cercariae from annelids are referred to Parvatrema borealis and the heavy infection suggests that polychaete annelids are natural hosts and that shore birds which feed on them may be definitive hosts.