The existence of a direct link between intensity of sexual selection and mating system type is widely accepted. However, the quantification of sexual selection has proven problematical. Several measures of sexual selection have been proposed, including the operational sex ratio (OSR), breeding sex ratio (BSR) and opportunity for sexual selection (Imates). For a wild population of pronghorn (Antilocapra americana), we calculated OSR and BSR. We estimated Imates from census data on the spatial and temporal distribution of receptive females in rut and from a multi-generational genetic pedigree. OSR and BSR indicated weak sexual selection on males, but census and pedigree Imates suggested stronger sexual selection on males than on females. OSR and BSR correlated with census but not pedigree estimates of Imates, and census Imates did not correlate with pedigree estimates. This suggests that the behavioral mating system, as deduced from the spatial and temporal distribution of females, does not predict the genetic mating system of pronghorn. The differences we observed between estimators were primarily due to female mate sampling and choice, and the sex ratio. For most species, behavioral data is not perfectly accurate, and therefore will be an insufficient alternative to using multi-generational pedigrees to quantify sexual selection.