Of 128 seasnails (Liparis atlanticus) collected from tide pools along the New Hampshire coast, over 90% were infected with the cestode Spathebothrium simplex and with one or both of the trematodes Podocotyle reflexa and Podocotyle atomon. Additional helminths found included Echinorhynchus gadi, Prosorhynchus crucibulum, and larval Thynnascaris sp. in 21, 12, and 9 percent of the hosts, respectively. All fish were infected with a species of Eimeria, and almost 30% had Trichodina sp. on their gills. The microscopic lesions associated with these infections are described, and the possible effects of these parasites on populations of L. atlanticus are discussed.