Meadow voles (Microtus pennsylvanicus) were trapped in the vicinity of Three Mile Island Nuclear Power Station between 6 and 16 April 1979. Thyroids of voles caught 1.9 km from the reactor (Site 111) contained significantly higher amounts of I3'I than those of voles caught further away. This is in agreement with Dept. of Energy predictions that this site was contaminated to a greater degree than the other two sites sampled. The highest level of I3'I detected from Site 111 was 11.4 pCi/thyroid and the mean for that site was 5.6 pcilthyroid. The vole is proposed as a monitoring organism for I 3 l I contamination of ecosystems.