This northern species is known from Ind., Ohio and Va., but extensions south to Ky. and N.C. remain rather uncertain (FNA 2, W). There is a coll. of Cranfill (1980) from FLEM (KY) that has been determined as this taxon by H. Wagner. However, distinction of this coll. from pycnostichum remains somewhat dubious. In pusillum, sterile blades are elliptic (versus ovate, broadest below middle), pale dull green (versus dark shiny green, with firmer texture); basal frond sheaths are membranaceous and ephemeral (versus leathery and tending to persist); spores are 50-60 microns wide (versus 35-45 microns); 2n = 960 (versus reportedly ca. 480-1320).