Peromyscus californicus are exclusively monogamous in the wild. We examined in the laboratory whether established pairs of male and female P. californicus would remain faithful when given the opportunity to mate with an opposite sex stranger, either in the presence or absence of their partner. When the partner was present she or he was tethered and could not intervene. Females in postpartum estrus (day 0 postpartum) preferred to associate with their pairmate rather than the unfamiliar virgin male. Females also preferred to mate with their partner, but 15–20% of the females mated with the unfamiliar male both in the presence and absence of their partner. Paired males were tested with an unfamiliar, sexually inexperienced estrous female on days 4 and 8 postpartum with their partners either absent or present but not in postpartum estrus, and on day 0 in the absence of their partner. Males preferred to associate with their pairmate. Contrary to theoretical expectations, males did not copulate with the estrous female when given the opportunity regardless of whether their partner was present or not. Furthermore, few males (2/13) tested in the absence of their partner mated with a sexually experienced, postpartum estrous female. These results indicate that monogamy in P. californicus is maintained by a strong attraction and preference of pairmates for each other and by self-restraint from mating with others. Male mate guarding may further ensure female faithfulness. Males exhibit apparently more sexual fidelity than females.