A systematic study of the chewing louse genus Acidoproctus (Phthiraptera: Ischnocera: Philopteridae) clarifies the identity of specimens placed as A. moschatae (L.) in the K. C. Emerson Collection. The described species are reviewed and A. kelloggi Carriker is placed as a synonym of A. moschatae. Acidoproctus granthami Arnold, new species is described from the freckled duck (Stictonetta naevosa (Gould)) from Australia. The chewing louse genus Acidoproctus (Phthiraptera: Philopteridae) includes 9 described species (Price et al., 2003) which are found on ducks and geese of the families Anatidae and Anseranatidae (Aves). The genus is worldwide, found wherever their hosts occur. Lice of this genus in the K. C. Emerson Collection at Oklahoma State University include two forms from different hosts, both identified as A. moschatae (L.). A comparison of these specimens with other specimens from the National Museum of Natural History (Washington, DC) and The Natural History Museum (London) resulted in the identi- fication of a new species. Herein I provide redescriptions and illustrations of the described species, place one species in synonymy, and describe and illustrate a new species. Synonymy follows Price et al. (2003) except that A. kelloggi Carriker is placed as a synonym of A. moschatae (L.). Host data follow Dickinson (2003). Materials and Methods All specimens were mounted on microscope slides and examined with a compound microscope which included an ocular micrometer. In the following descriptions, all mea- surements are in millimeters. Abbreviations for dimensions are HW (head width at temple), HL (head length at midline, but including the frontal projections), PW (prothorax width), MW (metathorax width), AW (width of abdomen at segment III), TL (total length), and PL (length of male paramere). The value in parenthesis following a statement of range is the mean.