To increase the efficiency of Syrian hamster reproduction, individual males were caged with 2 or 4 estrous females. The fertility of the males caged with 2 or 4 females was lower than that of those caged with a single estrous female. To clarify the cause of this, we observed the copulating behavior of males for 30 minutes and examined their fertility. The median frequency of mounting, intromission and ejaculation was 27, 36 and 3, respectively, for the males caged with a single female, 37, 27 and 4, respectively, for the males caged with 2 females and 13, 2 and 0, respectively, for the males caged with 4 females. Thus, these parameters were markedly reduced in the 1:4 mating group as compared with the 1:1 and 1:2 mating groups. In the 1:2 mating group, spermatozoa were detected in the vaginas of 12 of the 14 females, but only 6 of these 12 females delivered litters. In the 1:4 mating group, complete sequences of copulating behaviors were rarely seen, and spermatozoa were detected in the vaginas of only 6 of the 28 females, and only one of these 6 females delivered. Thus, the efficiency of offspring production was not increased by raising the number of estrous females exposed to a male, probably due to a decrease in the frequency of male copulation in the presence of 2 females and due to inhibition of the male's copulating behavior in the presence of 4 females.