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Large differences in conception rate during pasture-mating can exist among beef bulls having semen of acceptable quality. Based on data from artificial insemination, we know also that conception rate increases as the number of inseminations increases, and it is probable that conception rate to natural mating increases as number of matings per cow increases. Sexual aggressiveness (libido) is a behavioral factor that can influence the number of matings a bull can achieve and, thus, may influence the conception rate obtained during natural mating. Accurate evaluation of sexual aggressiveness and its relationship to fertility may provide useful information for selecting young beef bulls with high breeding potential. We conducted the following studies to investigate sexual activity in young beef bulls during a series of libido tests and to evaluate the relationship between libido test activity and fertility during natural mating.