The frequency of interactions among individuals is vital to understanding many aspects of a species' behavioral ecology. This also is a fundamental aspect of infectious disease epidemiology. However, the number of times an individual animal comes into contact with another is an extremely difficult parameter to estimate. This paper describes radiocollars (proximity detectors) capable of recording close contacts among radiocollared individuals. We conducted laboratory tests using collars alone (i.e., not fitted on an organism) and field tests of collar performance with collars fitted on 42 free-ranging raccoons (Procyon lotor) within a 20-ha portion of the Ned Brown Forest Preserve in northeastern Illinois, USA. During laboratory tests, we determined variation in detection distance within and among collars, effect of collar orientation on detection distances, and accuracy of recorded duration of proximity and identification of contacted collar. Overall variation in detection distance both within and among collars was relatively low, although we observed intrinsic variation in collar strength. We found little variation in horizontal detection distances with collar orientation, although mean distances for vertical tests were greater than most horizontal means. Recorded contact duration deviated from actual time by 3 seconds for short-duration (10-300 sec), and by 30 seconds for extended-duration (8-14 hr) contacts recorded as a single event. However, there was a tendency for the collars to record extended-duration contacts as multiple events, with the frequency dependent on settings. Identification number of the contacted collar was correctly identified in all cases (n ¼ 1,537). We downloaded 35 of the 42 proximity detectors deployed on free-ranging raccoons. Of these, approximately 57% were functioning properly, 9% exhibited problems apparently correctable in the field, and 34% exhibited problems not correctable in the field. For random pairs of raccoons, daily contact duration and number of contacts (omitting 1-sec contacts) recorded by both detectors did not differ (P ! 0.066). Overall, collars produced accurate information in terms of detection range, duration of contact, and contacted collar identification, and represented a promising improvement over previous methods to assess intraspecific contact rates of secretive, nocturnal, or otherwise unobservable species. Because accurate determination of contact rates requires that a population be saturated with detectors, their use may be most appropriate for species readily trapped. (WILDLIFE SOCIETY BULLETIN 34(5):1333-1344 2006)