Female hamsters freely interacting with a triad of males which had established a dominance hierarchy mated first and received more ejaculates from the dominant male in 11 of 15 replicates. Continuous observations of the animals on the day preceding receptivity and the day of receptivity suggest that differential mating success of the dominant males is the result of differential behavioral responses displayed prior to onset of receptivity. The females attacked all males, however, the dominant males persisted in following and licking the female at significantly higher frequencies than the subordinate males. The females responded differentially to the dominant males by vaginal marking in response to being licked and also on day 3 of the cycle by placing a higher number of vaginal marks in and around the dominant male's home area. In nine replicates the dominant male and female slept together on day 3. In two other cases the dominant male dug open the female's burrow early on day 4. Dominant males and females became active significantly earlier than subordinate males on day 4. Thus, dominant males enhanced their chances of mating both by being active earlier and by sleeping in proximity to the female. Males cannot prevent the promiscuous female from mating with another male. However, successful males continued to follow and mount the female during the period when she is likely to accept a second male. At this time they also display a high level of attack and chases against the subordinate males plus a high frequency of flank marking in the subordinate's home areas. These behaviors suggest a mate guarding function and are consistent with protection of the male's sperm investment.