The social behavior of the smaller fox species is poorly known compared with that of larger and more gregarious canids that can be directly observed. We studied social relationships and reproductive behavior of island foxes (Urocyon littoralis) on San Clemente Island (SCI) and San Miguel Island (SMI), off the coast of Southern California, using proximity logging collars. On both islands, pair members interacted more frequently and spent more time together than other types of dyads. On SCI, a high contact rate identified the same 10 mated pairs that were identified by a high degree of home-range overlap. A high contact rate also identified 3 mated pairs on SMI, 2 of which were associated with large pups 9–10 months old. On SMI, the number of contacts and amount of time that mates spent together per day varied across months and peaked in February, when most conceptions occur. Mates spent much less time together in April to June, the pup-rearing season. More unpaired male–female and male–male dyads interacted during January and February than in subsequent months. Paired males appeared to guard their mate during the period when she was receptive. The average duration of estrus was approximately 40 h. Paired females were in contact with other males in addition to their mate near the time they were presumably in estrus, and paired males were in contact with other females in addition to their mate. The relatively short duration of estrus, combined with asynchrony of estrus among females, enabled paired males to leave their mate and seek extrapair copulations without risk of cuckoldry. Proximity logging collars provide a new opportunity to learn about these temporal aspects of social and reproductive behavior in canids.