We summarized data from 2,479 intensive 2-hr morning surveys and 950 general surveys (< 2 hr duration) between 1988 and 1994 to determine the distribution of marbled murrelets (Brachyramphus marmoratus) in mature and old-growth forest stands on the Siskiyou National Forest of southwestern Oregon. Marbled murrelets were detected on 449 of these surveys. Murrelets were found in Douglas-fir (Pseudotsuga menziesii) or coast redwood (Sequoia sempervirens) stands within the western hemlock (Tsuga heterophylla) vegetation zone on the western part of the Forest. Murrelets were not detected in the mixed conifer/mixed evergreen zone on the eastern part of the Forest. This zone, which extends from 20-60 km inland, is drier and has smaller trees than the western hemlock vegetation zone. The distribution of murrelets on the Siskiyou National Forest may be limited by the availability of trees with suitable nesting limbs. Marbled murrelets (Brachyramphus marmoratus) are closely associated with older-aged coniferous forests (Marshall 1988, Hamer 1995, Hamer and Nelson 1995a, Miller and Ralph 1995). Unlike most species of the family Alcidae, marbled murrelets most often nest in trees. Sixty-five tree nests have been found in North America (Hamer and Nelson 1995a). Stands used for nesting by murrelets usually have large-diameter trees, relatively low canopy closure, and high numbers of potential nesting platforms (Grenier and Nelson 1995, Hamer and Nelson 1995a). The marbled murrelet was listed as a threatened species in Washington, Oregon and California (U.S. Fish and Wildlife Service 1992) primarily because of habitat loss from logging and development. Currently, surveys for marbled murrelets are mandatory for all proposed activities (primarily timber harvest) on federal lands within 80 km of the ocean that may affect their potential (old-growth and mature) habitat. Murrelets primarily travel up to 84 km inland in search of suitable nest sites (Carter and Sealy 1986, Paton and Ralph 1990, Nelson et al. 1992, Hamer 1995, Hamer and Nelson 1995a), although sightings are known from as far as 108 km inland in Oregon, and 101 km in British Columbia (Rodway et al. 1992; S. K. Nelson, pers. comm.). S. K. Nelson (unpubl. data) and P. W. C. Paton (unpubl. data) conducted limited surveysfor murrelets on the Siskiyou National Forest between 1988 and 1990. They found murrelets as far as 42 km inland. Additional information was needed on murrelet distribution in the coastal forests of the Siskiyou Mountains to aid in habitat management. Therefore, between 1991 and 1994 we conducted additional surveys for murrelets. The purpose of this study was to summarize marbled murrelet distribution from existing information and our surveys, and relate its presence to general habitat features. Because marbled murrelets nest primarily in large trees with large moss-covered limbs and they appear more abundant close to the coast (Hamer and Nelson 1995a), we hypothesized that factors such as habitat suitability, vegetation type, topography, climate (including temperature and relative humidity), and distance from the ocean would limit murrelet distribution.