I used bat detectors to study the effects of a fuels-reduction silvicultural treatment in a forested area of northeastern Oregon where there were high numbers of standing and down dead trees. Data were collected in 3 control and 6 treatment units over a 4-wk period in mid-summer for 3 y prior to treatment in 2002 and for 4 y after treatment. Treatment significantly reduced number and basal area of tree stems and canopy layers. Bat species were divided into clutter-intolerant, clutter-tolerant, and clutter-neutral species groups based on morphologic and echolocation call characteristics. The proportion of each group's activity did not differ between control and treatment units prior to treatment and for control units pre- and post-treatment. However, the proportion of activity of clutter-intolerant species was greater in treatment units after treatment compared to treatment units prior to treatment and to control units after treatment. The opposite result occurred for the clutter-tolerant species group. No significant changes were observed for the clutter-neutral group. There was no change in overall bat activity after treatment. I attributed changes in activity among the groups primarily to the reduction of clutter, but at least some of the change may have been an indirect effect of the treatment on the availability of prey or roost sites.