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Little is known of the mating system of the swift fox or how it compares to other socially monogamous mammals. In a 4-year study of 188 swift foxes, we used microsatellite analysis at 11 loci along with spatial observations to investigate swift fox mating strategies. The mating strategies used by swift foxes were highly diverse. Previous field observations have indicated that the swift fox is socially monogamous. However, we found that extrapair mating was a common breeding strategy; 52% of offspring were sired by a male that was not the mate of their mother. There was also variation in the structure of social groups. Of 59 social groups, the most common consisted of a male and female pair (93% of social groups); however, four stable trios of both one female and two males (5%) and two females and one male (2%) were also evident. The trio groups were spatially associated, and at least one member of each trio was highly related to a member in at least one other trio. Swift foxes also engaged in mate switching, which refutes the prevailing hypothesis that they always mate for life. Thus, we found that the mating system of the swift fox is highly diverse and substantially more complex than previously believed. We discuss factors that may influence which strategies are adopted and whether they are adaptive.