Based on partial autocorrelation analysis, 20 ermine (Mustela erminea) populations in Manitoba, Ontario, and Quebec demonstrated cyclic dynamics characterized by a latitudinal gradient of decreasing first-order feedback and increasing negativity of second-order feedback. Most of these populations exhibited three cyclic peaks and a 10-year interval of noncyclic dynamics during the sampling period (1915-1940). Changes in ermine density probably reflected those in the density of microtine rodents, their primary prey. Analysis of the limited number of long-term lemming and vole series from boreal North America indicated a latitudinal gradient in cyclic dynamics similar to that of microtine rodent populations in northern Europe. Complex geographic and temporal variation in ermine population dynamics, including cyclic, noncyclic, and shifting patterns of density change, supports the specialist-generalist hypothesis of predator-prey interaction at temperate latitudes.