The northern portion of the range of Ambrysus mormon is redescribed based on collection records from national and state biomonitoring databases and other sources. The northern range limit of this species includes the Klamath River drainage along the California—Oregon border; the Middle Snake River drainage in eastern Oregon; southern and central Idaho; all of Montana except the northwest corner; North Dakota and South Dakota, west of the Missouri River; and Nebraska, north of the Platte River and west to the Missouri River.