Knowing kill rate is essential for knowing the basic nature of predation. We compared estimates of kill rate for previously observed wolf-prey systems with new observations from wolves Canis lupus which preyed on white-tailed deer Odocoileus virginianus in Michigan, USA. For the five packs that we studied during 2001-2004, the mean kill rate was 0.68 kill/pack/day (∼7.7 kg/wolf/day). However, kill rates varied considerably. In particular, the coefficient of variation associated with the means was 0.55 for kills/pack/day and 0.68 for kg/wolf/day. Our analysis of previously observed kill rates also revealed a negative correlation between the duration of observation and the estimated kill rate. This correlation is the basis for showing how most published estimates of kill rate for wolves during winter tend to overestimate, by 50%, the season-long average kill rate during winter. The negative association between duration of observation and estimated kill rate occurs, in part, because wolves are unable to maintain very high kill rates for a long time. We also document how estimates of kill rate based on ground tracking tended to be 3.3 times greater than aerial-based estimates (2.4 vs 7.9 kg/wolf/day). Ground tracking is better able to detect multiple carcasses at one site, and better able to detect carcasses when wolves bed far from their kills. These previously undocumented biases are surprising given that wolves are so extensively studied.