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A medium-size (3.7-4.1 cm wingspan) light yellow-grey or grey-brown moth, frequently with a scattering of yellow scaling in the basal area and around the spots. The antemedian, median and postmedian lines often appear smudged, and the orbicular and reniform are poorly marked or nearly obsolete, or indicated by a patch of yellowish scales. The postmedian line is separate from the median line, and scalloped at the veins. The fringe is concolorous with the rest of the forewing. The hindwings are light brown or grey, with a small discal bar and pale fringes. Similar to some specimens of E. comosa. Males can be separated by the shorter saccular extensions (2/3 the length of the harpes in vallus, as long or longer in comosa), and females by the absence of sclerotized processes on the apex of the ovipositor lobes (present in comosa). Euxoa vallus belongs to the subgenus Pleonectopoda, characterized by the prominent twist or subbasal coil in the vesica of the male. There are no characters that can be used to identify females as members of the subgenus. Keys to the subgenus and species are presented in Lafontaine, 1987.

The nominate subspecies, which is the form that occurs in Alberta, is found in the mountains of southern Alberta, southern British Columbia and Washington. In Alberta it has been collected from the Bow valley north to the Cardinal Divide.