The design and testing of an electronic device, MateID (Electronics Laboratory, Department of Conservation, Wellington, New Zealand), for recording close proximity between wild animals is described. Comprising a receiver and data logger in a collar mounting, the device detects period-coded radiotags at distances below 40 cm. It stores time-stamped identification data in memory for up to 2 years for later down-loading to computer for processing. Bench trials of the MateID device confirmed its reliability. It recorded presence of 98.9% of stationary single tags placed within 40 cm and identified all of these correctly. However, no tags were identified where ≥2 tags were present within a 40-cm range. Trials on MateID devices attached to neck collars on captive brushtail possums (Trichosurus vulpecular) were conducted in large pens. Simultaneous video recordings confirmed that the devices accurately recorded all observed approaches (closer than 40 cm) by possums carrying MateID collars to a bait feeder fitted with a radiotransmitter. Contacts between a radiotagged possum and three carrying a MateID device also were accurately recorded. The device “MateID” provides a new technique for studying social and mating systems of free-ranging vertebrates. It also can be useful for monitoring or research of captive animals.