The accuracy of microhistological techniques to describe herbivore diets can be nffected by differential digestibility of ingested forages. Correction factors were developed to adjust for those effects in 17 common forages of coastal, forested ranges of the Pacific Northwest. Two ferns, a moss and a sedge were overestimated by microhistological analysis in nil seasons, while most shrubs, forbs and a grass were underestimated. Trees were not consistently overor underestimated. Phenology significantly affected the degree of overor underestimation of most forages. Failure to correct for differential digestibility will significantly bias results of microhistological techniques such as fecal analyses. Microtechniques for determining food habits of large herbivores can be biased by differential digestibility of ingested plant species (Holechek et al. 1982). For example, the accuracy of fecal analysis can be affected by the extent of digestion of plant epidermis as it passes through the alimentary tract of a ruminant (Steward 1970, Slater and Jones 1971, McInnis et al. 1983) and by sample preparation techniques (Vavra and Holechek 1980). Yet, fecal analysis is used widely for describing diets of wild and domestic herbivores (e.g., Free et al. 1970, Stewart and Stewart 1980, Hansen and Martin 1973, Hansen et al. 1973, Todd and Hansen 1973, Anthony and Smith 1974). Frequently, it is the only practical method available (Vavra et al. 1978), particularly when dense vegetation and wariness of study animals preclude direct feeding observations and when protection from hunting of some wild and/ or rare herbivores eliminates the possibility of collecting ruminal samples. Also, fecal material usually is readily available, which enables the collection of an adequate number of samples at any time of the year. Results from fecal analyses can be improved by species-specific correction factors that compensate for differential digestibility of ingested forages (Voth and Black 1973, Dearden et al. 1975, Fitzgerald and Waddington 1979, Pulliam and Nelson 1979, Vavra and Holechek 1980). Forbs are usually highly digestible and as a result, underestimated by fecal analyses (Vavra et al. 1978, Vavra and Holechek 1980, McInnis et al. 1983). Some grass and browse species are overestimated by fecal analyses, while others are underestimated (Dearden et al. 1975, Vavra and Holechek 1980). Phenology affects digestibility of most forages (Laycock and Price 1980) and thus, season-specific correction factors may be necessary for individual plant species in the diet (Pulliam and Nelson 1979). Authors ate graduate research assistant, Cooperative Park Studies Unit and Department of Fisheries and Wildlife; professor, Department of Rangeland Resowces, Eastern Oregon Agricultural Research Center; Unit Leader, Cooperative Park Studies Unit, School of Forestry; laboratory assistant, Eastern Oregon Agricultural Research Center, respectively; all associated with Oregon State University, Cot’vahis, 9733 I. Dr. Leslie is currently assistant professor of Wildlife Science, Division of Wildlife College of Forest Resources, University of Maine, Orono, ME 04469. ‘fhts research was funded by the National Park Servtce (Contract No. CX 9ooo-O-E935) through the Cooperative Park Studies Unit at Oregon State University. Authors thank R.G. Anthony and K.J. Jenkins for helpful comments on earlier drafts of this paper. Manuscript received November IS, 1982. Unfortunately, these relationships have been described for relatively few forages. This report provides correction factors for common forages of coastal, forested ranges of the Pacific Northwest that are important to both wild and domestic ungulates.