Radiotelemetry is an important tool for wildlife management and research, but in some cases attachment of neck collars can be problematic. An alternative in large mammals is to attach transmitters to the ear, though little is published about ear-tag radiotransmitter integrity (i.e., how long a transmitter emits a useful signal) and retention (i.e., how long a transmitter remains attached to an animal). Here we report ear-tag transmitter integrity and retention from 2 studies monitoring free-ranging calves (Bos taurus) in eastern Arizona, USA, and feral horse (Equus ferus) foals in northwestern Spain. Transmitter integrity and retention was lower for transmitters attached to foals then calves. The primary cause for reduced integrity was antennas breaking off, whereas the primary retention problem involved transmitters ripping out of the ear. When data were pooled across study sites, mean integrity and retention loss was 111 days and 180 days, respectively. Transmitters attached to the interior of the outer ears had retention rates >2 times higher than transmitters attached to the exterior of the outer ear (88% vs. 43%). We recommend that researchers intending to utilize ear-tag transmitters for studies on large domestic or wild animals attach transmitters to the interior of the outer ear, reinforce transmitter antennas in order to improve integrity, and report integrity and retention rates. © 2012 The Wildlife Society.