We conducted surveys for Hesperopsis gracielae (MacNeill) along the lower Colorado River floodplain from the Muddy River inflow of Lake Mead in Nevada south through western Arizona and eastern California to the eastern border of Baja California, Mexico. Females oviposit largely if not exclusively on quail brush, Atriplex lentiformis. We searched for eggs, larvae, pupae, and adults on or around quail brush and nearby food sources. Eggs are distinctive by their spherical shape, heavily-sculptured and ridged chorion, and reddish brown or cream color. Larvae form and pupate in shelters that are easily distinguished from other Lepidoptera. Finding immature stages helped identify occupied locations, because eggs or larvae could be found between adult broods and during weather conditions not conducive for adult observations. We found quail brush at 99 sites along the floodplain and sootywings at 55 of those sites. Several large regions of the lower Colorado River did not appear to support quail brush. We did not find H. gracielae at the type locality at Bennett Wash near Earp, California, despite the presence of quail brush. The closest location to the type locality currently occupied is 21 km northeast along the Bill Williams River.