A new species of the trematode genus Maritrema, Maritrema arenaria, is described as to morphology, adult host (Arenaria interpres), and metacercarial host (Balanus balanoides). Negative results are reported from a search for the sporocyst and cercarial stages of this fluke, and the suggestion is made that the cercaria may not be present during the summer months in the Woods Hole region.