This is the 1st effort to determine the mating behavior of the Tehuantepec jackrabbit (Lepus flavigularis), an endangered endemic species from southern Mexico whose distribution is limited to the Isthmus of Tehuantepec region. Between June 2006 and December 2007, observations of 60 adult radio-marked jackrabbits were carried out in Santa María del Mar, Oaxaca, Mexico. Direct observations showed that the breeding season of the Tehuantepec jackrabbit lasted an average of 200 days per year, with a high-intensity period during the rainy season (May–October); we also confirmed a polygamous reproductive behavior for this species. More importantly, we showed that the Tehuantepec jackrabbit has a polygynous mating behavior. In a single night, a male courted and mounted an average of 2 females; whereas during the whole reproductive season, the male courted an average of 6 females and mounted 3 of them. In a single night, a female was courted by an average of 3 males but mounted by only 1 of them. Throughout the whole reproductive season, the female was courted by 5 males but mounted only by 1 of them. This is the 1st research concerning mating behavior of the Tehuantepec jackrabbit; the resultant information will be very useful for the jackrabbit's conservation, management, and reproduction in captivity.