Aspidogaster conchicola infection is reported in 2 species of freshwater gastropods, Viviparus malleatus and V. japonicus. This represents the first report of this trematode from Massachusetts and also the first time that gastropods have been implicated as hosts in North America. Observations are presented on natural infections and their pathology in the snails, in vitro maintenance of A. conchicola for periods up to 135 days, and morphologic variations of this trematode's opisthaptor. Aspidogaster conchicola, a common parasite of freshwater mollusks, has been reported from China, North Africa, Central Europe, and North America (Dollfus, 1958). Although it has been found occasionally in species of freshwater gastropods, fish, and turtles (Faust, 1922; Simer, 1929; Dollfus, 1958), infections in animals other than Unionidae have been considered accidental and transient (Van Cleave and Williams, 1943). It was of interest, therefore, to find that A. conchicola frequently parasitizes the snails Viviparus malleatus and V. japonicus in the Boston area. This represents the first report of A. conchicola from Massachusetts and also the first time that gastropods have been implicated as hosts in North America. Observations are also presented on natural infections and their pathology in snails, the in vitro maintenance of A. conchicola, and on peculiarities in the morphology of this organism. MATERIALS AND METHODS Viviparus malleatus were collected from Emmanuel Pond and from the Muddy River, Brookline, Massachusetts, and V. japonicus from Spot Pond, Blue Hills, Massachusetts. Sixty specimens of the former and 10 of the latter were used in this study. No unionids were found in either of the collecting sites. Whole mounts of A. conchicola were prepared from specimens fixed in AFA and stained with Delafield's hematoxylin or Mayer's acetic carmine. Other specimens were fixed in Zenker's 5% acetic acid solution, embedded in paraffin, sectioned serially (8 4), and stained with hemalum and azure II-eosin. Snails were fixed in 10% neutral formalin, embedded in paraffin, sectioned Received for publication 26 August 1969. * These studies were supported, in part, by Grants AI-00513 and AI-00046 from the NIAID, U. S. Public Health Service. serially (8 4z), and stained with hemalum and eosin, hemalum and azure II-eosin, or Gomori's trichrome. To study the opisthaptor, living worms were relaxed with menthol crystals, fixed in hot AFA, and stained with half-strength Lugol's iodine. The worms were then oriented, opisthaptor up, in a small drop of 70% alcohol on a slide. Details of the opisthaptor were observed using a stereomicroscope ( X 30). Trematodes were maintained axenically in glass roller tubes (20 by 150 mm) containing 5 ml of a modified Unionid Ringer's Solution (Ellis et al., 1931). The solution was modified by adding 5% clam blood (sterile, from Anodonta implicata), penicillin G (100 units/ml), streptomycin sulfate (100 jg/ml), and sufficient sodium bicarbonate to yield pH 7.9. Tubes were sealed with nonreactive rubber stoppers, but opened every 10 days. Groups of 10 tubes, each containing one adult worm, were maintained at 4, 20, and 25 C in each of two experiments.