Fresh, liquid-preserved, air-dried, and critical-point dried materials of Splachnum pensylvanicum from New York State, U.S.A., examined by light and scanning electron microscopy, revealed the following. The plants are autoicous; the spores are minutely reticulate; the apophysis is translucent, yellow-green, and wider than the urn when fresh and green but more opaque when dry; the operculum is blunt and rounded when fresh but mamillate when dry; the inner surfaces of peristome teeth are papillose, while the sculpturing of the outer faces is in parallel sinuose rows; the spore sac seems to participate in spore presentation at the mouth of the urn; setae grow about three times in length during spore release; and polysety. Distinctions between this moss and Tetraplodon angustatus in the shape of the capsule and operculuin are clarified. In S. pensylvanicum the apophysis is wider than the urn when fresh but narrower when dry; in T. angustatus it is wider in fresh and dry plants. In S. pensylvanicum the operculum apex is broad and rounded when fresh (and similar to that of T. angustatus) but mamillate when dry. Vegetative leaves of S. pensylvanicum and S. ampullaceum, which are found at the base of the plants often among the rhizoids, are similar in shape and dentation but differ somewhat in areolation and in the degree of tapering toward the leaf base.