Agonistic interactions and receptivity of females during mating encounters in the corn mouse (Calomys musculinus) were studied. Twenty-nine couples were observed for copulatory behavior during postpartum estrus; 14 of them were composed of males and females that had bred together at least once (familiar male group), and 15 were composed of a male and a female each with reproductive experience, but were unknown to each other (strange male group). No significant differences between the familiar male and the strange male group were found in either the mount latency or the intromission latency. The strange male group showed a trend to have higher mount and intromission frequencies, both at the first and the second ejaculatory series, although some of these differences did not reach statistical significance. Also, males in the strange male group showed significantly higher ejaculatory thrust frequencies, both at the first and the second ejaculatory series, than males in the familiar male group. The post-ejaculatory interval was significantly longer in the strange male group. The incidence of all agonistic behaviors other than a stereotyped kind of fighting that precedes copulation (“precopulatory fighting”) was higher in strange male group than in the familiar male group. In both groups, aggressive postures were more frequently seen in females than in males, and conversely, submissive postures were significantly more frequent in males than in females. Also, females in the strange male group made significantly more approaches than those in the familiar male group. Taken together, these findings suggest that C. musculinus is a non-monogamous species.