Yellowstone bison Bison bison are chronically infected with brucellosis (caused by the bacterium Brucella abortus), which raises concerns about possible transmission to cattle when they migrate to winter ranges outside the Yellowstone National Park. We monitored bison from April to mid-June during 2004-2007 to estimate the timing and location of parturition events that may shed tissues infected by B. abortus. Observed abortions (N = 29) occurred from January through 19 May, while peak calving (80% of births) occurred from 25 April to 26 May, and calving was finished by 5 June. Observed parturition events (N = 115) occurred in the Park and on the Horse Butte peninsula in Montana, USA, where cattle were not present at any time of the year. Allowing bison to occupy public lands outside the Park where cattle are never present (e.g. Horse Butte peninsula) until most bison calving is completed (late May or early June) is not expected to significantly increase the risk of brucellosis transmission from bison to cattle because: 1) bison parturition is essentially completed weeks before cattle occupy nearby ranges, 2) female bison meticulously consume birthing tissues, 3) ultraviolet light and heat degrade B. abortus on tissues, vegetation and soil, 4) scavengers remove fetuses and remaining birth tissues and 5) management maintains separation between bison and cattle on nearby ranges. Allowing bison to occupy public lands outside the Park through their calving season will help conserve bison migratory behaviour and reduce stress on pregnant females and their newborn calves, while still minimizing the risk of brucellosis transmission to cattle.