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Undergrowth vegetation response to fuel reduction treatments was tested in fire-adapted ponderosa pine (Pinus ponderosa)/Douglas-fir (Pseudotsuga menziesii) forests. Treatments included: no treatment, prescribed burning, low thinning, and low thinning followed by prescribed burning. Direct effects of the fuel reduction treatments were observed on the undergrowth vegetation the first growing season after burning and three seasons after thinning. Burn-only treatments tended to reduce the diversity of the undergrowth and diminish the cover of grasses and shrubs while augmenting the frequency of fire adapted undergrowth species. Thin-only treatments had very little impact on species diversity but graminoids and shrubs tended to increase cover. Fire sensitive species were able to increase frequency in the Thin-only units. The Thin-and-Burn treatments elicited a response similar to that of the Burn-only treatments, though there was some indication that the fire may have been more intense, thereby magnifying the effect of burning. i