The family Paeromopodidae is comprised of large, agile millipeds that include the longest diplopods in the Nearctic. It is endemic to three regions in the western United States - a large, irregular area extending from the southern Sierra Nevada Mountains and the Monterey Peninsula of California to the northern Puget Sound region of Washington, a small, ovoid area in the Warner Mountains of northeastern California and the adjacent fringes of Oregon and Nevada, and an area in the northwestern interior extending from the Wallowa Mountains of northeastern Oregon to Flathead Lake, Montana. Representatives have flat, plate-like gonopods that represent the coxites; the flagellum is absent from the anterior gonopods and the telopodite is lost from both gonopod pairs. The family includes two genera: Paeromopus Karsch, with five species - angusticeps (Wood), eldoradus, cavicolens, and buttensis, all by Chamberlin, and ocellatus Loomis - and Californiulus Verhoeff, with five species - chamberlini (Brolemann), comb. n., dorsovittatus Verhoeff, yosemitensis Chamberlin, euphanus (Chamberlin), and parvior (Chamberlin). The principal taxonomic features involve the configurations of the distal and midlength projections of the anterior gonopods and the terminal or subterminal projections of the posterior genitalia. Californiulus chamberlini, dorsovittatus, and yosemitensis display broad, yellow, middorsal stripes, the last also with a median black line, and the other species exhibit light or dark transverse bands. The following new synonymies are proposed: P. lysiopetalinus Karsch and pistus Chamberlin, and Paeromopellus sphinx Verhoeff, under P. angusticeps; Klansolus zantus Chamberlin under C. chamberlini; C. vicinus Chamberlin under C. dorsovittatus; K. mononus and obscurans, both by Chamberlin, under C. yosemitensis; and K. socius and Aigon rodocki, both by Chamberlin, under C. parvior. The monobasic subfamily Aprosphylosomatinae, accommodating Aprosphylosoma Hoffman, is elevated to family status in the superfamily Paeromopodoidea. Its sole species, A. darceneae Hoffman, known only from Oregon, is smallbodied, dark mottled brown in color, and has a separate coxite and telopodite on the anterior gonopod.