The Orthotrichum strangulatum complex in eastern and interior North America has three species: O. strangulatum P. Beauv., O. parvulum Mitt. and O. lescurii Austin. All three form blackish green, rigid tufts and are restricted to calcareous substrates. Orthotrichum strangulatum has oblong-lanceolate leaves with partially thickened laminae and multistratose, recurved margins. Its oblong-elongate capsules have 16, reticulate-papillose, free exostome teeth. Orthotrichum lescurii has a more narrowly lanceolate leaf shape than O. strangulatum with unistratose laminae and revolute margins. Its ovate capsules have eight pairs of finely papillose exostome teeth. Orthotrichum parvulum has oblong-lanceolate leaves with unistratose laminae and plane leaf margins. It has oblong capsules with 16, reticulate-papillose, free exostome teeth. Orthotrichum peckii Sull. & Lesq. in Austin, O. porteri Sull. & Lesq. in Austin, and O. strangulatum var. missouricum Holz. ex Grout are synonyms of O. strangulatum. Orthotrichum strangulatum var. missouricum is lectotypified with a specimen from Holzinger's herbarium in minn.