| abstract | Ornithonyssus hypertrichus n. sp. is described from four localities over a large area of the western nearctic region. Species characteristics include a high degree of hypertrichy on the opisthonotal portion of the female dorsal shield and the unarmed cuticule of the protonymph and adults, thus far unique within the genus. All specimens were collected from species of the cricetine rodent genus Peromyscus. A revised diagnosis is given for the genus Ornithonyssus. Keys are given to adult females, adult males, and protonymphs of the 6 species recognized as belonging to the genus Ornithonyssus in the nearctic region. |