Fisher (Martes pennanti) populations may respond numerically to fluctuating hare populations, although uncertainty remains about this contention. We analyzed livetrapping data from Ontario (448,654 trap nights) that contained trend information for fishers, lagomorphs (snowshoe hares [Lepus americanus] and eastern cottontails [Sylvilagus floridanus]), and North American porcupines (Erethizon dorsatum) for 1995–2004. The data encompassed a synchronous fluctuation of lagomorphs and porcupines. Fisher populations appeared to have a delayed, positive numerical response with a lag of 2 years to changing lagomorph abundance, but they also appeared to switch to an unmeasured, alternate prey during the lagomorph low. This finding suggests that fishers are generalist predators that should have a stabilizing effect on the predator–prey system.