Orthotrichum fenestratum Card. & Thér, described from St. Paul Island and reported only from a few small islands in the Bering Sea and Aleutian Chain, was considered unique because of perforations in the exostome. Comparisons with the uncommon, but widespread, arctic-montane species, O. pylaisii Brid., indicates that like O. fenestratum, O. pylaisii also has exostomial perforations and these species are conspecific. The distribution of O. pylaisii is mapped for northwestern North America. Orthotrichum sordidum Sull. & Lesq. was described from eastern North America and then reported from East Asia and from scattered locations in Alaska and Greenland. All of the Alaskan specimens are O. pylaisii and also all of the Greenland specimens examined belong to this latter species; consequently O. sordidum should be removed from the floras of these northern regions. Similar to O. pylaisii, O. sordidum also has perforate (and fenestrate) exostome teeth, but differs in a number of features, including 8 vs.16 exostome teeth; deeply ribbed, emergent capsules vs. lightly ribbed, exserted capsules; and 8, well-developed endostome segments vs. rudimentary segments of O. pylaisii. Orthotrichum sordidum occurs almost exclusively on tree trunks, while O. pylaisii occurs on rock.