Rocky Mountain dotted blue

''Euphilotes ancilla'', the Rocky Mountain dotted blue, is a species of butterfly of the family Lycaenidae. It is found from Washington south to California and southern Alberta and Saskatchewan south through the Rockies and high plains to Wyoming, Colorado, Utah, and northwestern New Mexico.

Wingspan is from 17 to 23 mm.

Larvae feed on various ''Eriogonum'' species, especially sulphur-flower.