Slightly larger and broader-winged than the Eastern Tailed Blue, with a grayer underside with smaller black dots and (often) stronger metallic scales near the base of the tail. This is a species of cool, moist habitats, from the Redwoods to the Sierran mid-West slope; it is completely absent from the Central Valley and typical E. comyntas habitats, and the two very rarely co-occur. (They may, however, hybridize where they do.) In the Sierra it occurs from 2500' upslope, and has a distinctive "ecotype" or ecological race above tree-line (e.g., on Castle Peak).

Like many Central Valley butterflies, this has been accused of being an introduction from somewhere else--but there is no good reason for thinking that it is. (We have done the molecular genetics which, alas, is uninformative.) The fact that it occurs widely and happily in disturbed habitats (in this case, annual grassland, riparian habitats and tule marsh) and often uses naturalized host plants contributes to the notion. It rarely turns up above 2000' and in our area is almost never sympatric with the Western Tailed Blue.