Of 41 adult wolf-killed gray wolves (Canis lupus) and 10 probably or possibly killed by wolves from 1968 through 2014 in the Superior National Forest (SNF) in northeastern Minnesota, most were killed in months leading up to and immediately following the breeding season, which was primarily February. This finding is similar to a published sample from Denali National Park, and the seasonality of intraspecific mortality generally parallels the known seasonality of testosterone levels, scent-marking, howling frequency, and general interpack aggression. Males and females were killed in the same proportion as in the population of radiocollared wolves. The annual rate of wolf-killed wolves was not related to the annual wolf density. Our findings tend to support intraspecific mortality of adult wolves as a means to reduce breeding competition and to maintain territories.