Several new names are proposed for a taxonomic revision of the dwarf mistletoes (Arceuthobium). Two new species from northwestern California and southwestern Oregon are described: A. siskiyouense, a parasite of knobcone pine, and A. monticola, a parasite of western white pine. Also described as new is A. littorum, a parasite of Monterey and bishop pines in coastal California. Arceuthobium tsugense is segregated into two subspecies: subspecies tsugense, primarily parasitic on western hemlock, and subspecies mertensianae, primarily parasitic on mountain hemlock. In preparing a revision of our monograph on dwarf mistletoes (Arceuthobium, Viscaceae) (Hawksworth & Wiens, 1972) and for our treatment of the Viscaceae for the new Jepson Manual of the Flowering Plants of California, we have continued to investigate the systematics of this economically important genus of conifer parasites (Hawksworth & Wiens, 1976, 1977, 1980, 1984, 1989; Nickrent, 1986, 1987; Nickrent & Butler, 1989, 1990, 1991; Nickrent & Stell, 1990; Nickrent et al., 1984). Here we describe three new species that parasitize pines in Oregon and California, and we also segregate the hemlock dwarf mistletoe into two subspecies. Two of the new species are endemic to the Siskiyou-Klamath Mountains floristic province of northwestern California and southwestern Oregon, an area well known for its high degree of plant endemism (Smith & Sawyer, 1988). Only brief descriptions of the new taxa, their relationships to closely related taxa, and selected collections representing the host and geographic distribution of the new taxa are cited here. Additional descriptions, illustrations, complete specimen citations, and distribution maps will be presented in our revised monograph of the genus, which is nearing completion. The methods used for our morphological NOVON 2: 204-211. 1992. analyses are as described by Hawksworth & Wiens (1972). Arceuthobium siskiyouense Hawksworth, Wiens & Nickrent, sp. nov. TYPE: U.S.A. Oregon: Josephine County, Oregon Mountain Road, 6 mi. SW of O'Brien, parasitic on Pinus attenuata, T. 41 S., R. 9 W., Sect. 9, 42?01'N, 123?46'W, elevation 650 m, 20 July 1987, D. Wiens 6756 (holotype, US; isotypes, FPF, MO, ORE, OSC, UC). Plantae 6-10 (8) cm altae; surculi brunnei, parce flabellate ramosi; surculi principales basi 2-2.5 mm diam., internodiis tertiis 6-15 (9) mm longis, 2 mm latis; flores staminati 3-4-meri; fructus maturi 4 mm longi, 2.5 mm lati; anthesis mense Septembri; fructus maturitas mense Septembri-Octobri. In Pino attenuata parasiticae. Plants 6-10 (mean 8) cm tall, brownish, flabellately branched, predominant shoots 2-2.5 mm diam. at base, third internode from base 6-15 (mean 9) mm long and 2 mm wide; staminate flowers 3or 4-partite; mature fruit 4 mm long and 2.5 mm wide; anthesis in September; fruits mature in September and October; parasitic principally on Pinus atten-