Avian cholera is one of the most important diseases affecting waterfowl in North America, but little is known about its ecology and its impact on waterfowl populations. We documented avian cholera mortality in breeding lesser snow geese (Chen c. caerulescens) at the Egg River colony on Banks Island, Northwest Territories, Canada, in 1995 and 1996. Area of the breeding colony, core nesting area, and number of nesting geese were greater in 1996 (colony=7,537 ha, core area=1,581 ha, 401,000 nesting geese) than in 1995 (colony=6,637 ha, core area=996 ha, 318,000 nesting geese). Density of nesting geese also was greater in the core area during 1995 (120 geese/ha) than in 1996 (90 geese/ha). Pasteurella multocida (serotype 1) was cultured from the leg bones of adult snow goose carcasses collected after outbreaks. Mortality from avian cholera began during nesting and continued until birds dispersed at hatch. Mortality appeared to be in foci scattered throughout the nesting colony, but generally was greater where there were greater densities of nesting geese. We estimated that 30,000 and 20,000 geese died in 1995 and 1996, respectively, about 5-9% of the nesting colony. Between 1991 and 1997, at least 4 avian cholera outbreaks occurred at Banks Island. It appears that avian cholera has become endemic in this population of snow geese, and these birds have the potential to transmit the disease to other waterfowl, especially on wintering areas where waterfowl are very concentrated.