In lek mating systems, females visit aggregations of displaying males and appear to have unrestricted opportunity to choose and mate with any male. Behavioral observations of lekking birds indicate that females generally mate once and that only a small percentage of males gain a majority of copulations. However, genetic analyses of paternity have revealed that multiple males sometimes sire young within one brood. We analyzed paternity of 25 broods of Greater Prairie-Chickens (Tympanuchus cupido pinnatus) breeding in an isolated population in central Wisconsin. We found multiple paternity in 44% of broods (1–4 sires brood^-1) and a positive relationship between the number of sires and female home-range size prior to nesting. Multiple paternity was apparently not influenced by local breeding experience: native Wisconsin and translocated females from Minnesota did not differ in the likelihood of multiple mating. The high incidence of multiple paternity may reduce the variance in male reproductive success and increase the effective population size, which may be important for the maintenance of genetic diversity in small, isolated populations.