Skeletochronology of sea turtle humeral bones has been validated only for juvenile Caretta caretta. Our recovery of a sea turtle marked with oxytetracycline (OTC) and at large for eight years shows that annual growth marks continue to be deposited in adult C. caretta. In addition, the presence of a fluorescent mark eight years after injection shows that OTC can be used in long-term growth studies. Growth and OTC marks are not equally distinguishable around the entire edge of the humerus making the location of bone biopsies critical for determining growth and OTC marks. Although bone biopsy sections may be used with OTC to count growth marks, they cannot be used to measure radial growth or to make back calculations of age. Instead, whole humeral sections must be used.