From 1984 through 1987, we studied aggressive responses of Prairie Falcons (Falco mexicanus) to species intruding into their nesting territories in southwestern Idaho (52 nesting attempts, 613 days, 9,085 hr). Prairie Falcons responded aggressively most frequently to Common Ravens (Corvus corax; 49% of encounters), followed by Red-tailed Hawks (Buteo jamaicensis; 24%), Golden Eagles (Aquila chrysaetos; 7%), Turkey Vultures (Cathartes aura; 4%), Northern Harriers (Circus cyaneus; 2%), American Kestrels (Falco sparverius; 1%), and bobcats (Lynx rufus; 1%). The frequency of aggressive responses toward intruders was similar for males and females, except in the case of American Kestrels and bobcats. Aggressive responses of nesting Prairie Falcons to intruders may be related to predator deterrence, competition for nest sites, stage of the nesting cycle, food availability, and sexual size dimorphism of falcons.