In 2001, I attached tail-mounted transmitters to 41 beaver (Castor canadensis) captured in Rocky Mountain National Park to evaluate if the transmitters (Rothmeyer et al. 2002) would be suitable to conduct long-term life history research. Twenty-one of the transmitters became detached from the tail without evidence of beaver mortality. I was unable to recover and determine the outcome for 9 transmitters because signals originated from the inside of active beaver lodges. Antennas were intact on 7 detached transmitters but were missing and likely chewed-off on 11 others. In conclusion, tail-mounted transmitters for beaver were of limited success in this life history study and are not recommended for long-term monitoring without significant modifications to the original design.