In Alligator mississippiensis (American Alligator), body length increases with age, but body length can be used as an accurate estimator of age only up to about 6–7 years, when growth rates slow considerably. Telomeres are repetitive DNA sequences that cap the ends of each chromosome. Telomeres shorten with age in most animals, but telomere shortening has not been examined in reptiles. We measured telomere length in erythrocytes of A. mississippiensis varying between ≈ 5 and 240 cm in body length and found a negative relationship between telomere length and body length (P < 0.01). Assuming that erythrocyte telomeres continue to shorten with time, even after growth rate declines, those individuals with the shortest telomeres should be the oldest members of the population. This method of estimating age, even in animals of similar body size, should allow questions about age structure and senescence to be addressed.