Wildlife population studies that involve marking of animals assume that the "mark" will remain with the animal for the duration of the study. Violations of this assumption may contribute to biased estimates of population parameters. Thirty-six gray squirrels (Sciurus carolinensis) and 16 fox squirrels (S. niger) were marked with ear tags and toe-clipped to study ear tag loss in these species. Fortynine percent of all squirrels lost at least 1 ear tag, whereas 15% lost both tags. Fox squirrels lost ear tags at twice the rate of gray squirrels. Female fox squirrels lost ear tags at a rate greater than gray squirrels and male fox squirrels. Studies that depend on identification of individual squirrels may be significantly affected by ear tag loss.