To effectively monitor winter foraging activity by the cryptic, non-site-faithful Virginia opossum (Didelphis virginiana), we tested the use of small data-logging temperature sensors (iButton Thermachrons^®, Maxim/Dallas SemiConductors, Dallas, Tex.) attached to a standard radiocollar on 3 opossums over the winter of 2000–2001. Two replicate sensors were required to clearly show time periods with cooler temperatures, an indication that the animal was outside the den. These foraging bouts were consistent with the available radiotelemetry data. Daily duration of foraging showed a strong negative relationship with ambient temperature, quantitatively documenting for the first time a phenomenon previously known only anecdotally. The iButton Thermachron seems to be an effective, low-cost, and low-effort technology for monitoring foraging activities of any animal that rests and forages in different temperature environments.