Horizontal stems mainly shallowly buried, 0.5--3 mm wide, sometimes emerging, 1.1--2.2 mm wide; leaves appressed, spatulate to lanceolate, 1.5--3.8 X 0.5--1.4 mm, apices truncate. Upright shoots 6--14 cm, clustered, fasciculate, branching successively 3--5 times; leaves on upright main stem ascending, deltate-ovate, 3.5--4 X 0.8 mm, apices needlelike. Branchlets square in cross section, 1.8--4 mm wide, annual bud constrictions abrupt and conspicuous; underside often glaucous, concave; upperside green, dull to faintly shiny, convex. Leaves on branchlets 4-ranked, overlapping; upperside leaves appressed, lanceolate, 3--5.8 mm, free portion of blades 1.7--2.9 X 0.1--1.1 mm; lateral leaves strongly divergent, 3.3--6.5 X 1.8--2.4 mm, margins revolute; underside leaves well developed, perpendicular to stem, 1.3--3.3 X 0.6--1.3 mm, unique in genus in having base contracted, blade flaring, and margins becoming parallel. Peduncles absent. Strobili solitary, 5--30 X 2--4 mm, sterile tips absent. Sporophylls deltate to nearly cordate, 2.2--3.5 X 1.6--3 mm, apices gradually tapering. 2 n = 46. The branchlet leaves of Diphasiastrum alpinum are unique in the genus, and the trowel-shaped underside leaves with their flared and rolled blades and contracted bases are particularly unusual. The leaves of the other North American species are much simpler in shape and contour.