During an accreditation review of the Shark Reef Aquarium at the Mandalay Bay Hotel in Las Vegas, Nevada, AZA (Association of Zoos and Aquariums) reviewers expressed concern about the possible effects of low light intensity and absence of ultraviolet light on serum 25-hydroxyvitamin D levels in our green turtles (Chelonia mydas). In an effort to address this concern, blood samples from the aquarium's 3 resident green turtles were periodically analyzed over a 5-year period. Serum 25-hydroxyvitamin D level in a green turtle living at a second indoor facility for 432 months was also analyzed. Two of 4 turtles housed at a third facility were moved from an outdoor to an indoor habitat, and serum 25-hydroxyvitamin D level of all 4 turtles was measured over a 2-year period. In this limited population, serum 25-hydroxyvitamin D levels began to decline about 4–5 months following indoor confinement and continued to decline for 6–8 years. Turtles in the outdoor facility had vitamin D₃ blood serum concentrations of 60–70 nmol/L. After 6–8 years of confinement indoors, blood serum concentrations of vitamin D₃ declined to 5–15 nmol/L. Although clinical symptoms of low serum 25-hydroxyvitamin D levels were not detected during regular veterinary examination of this limited study population, further research is needed to elucidate the long-term effects of restricted ultraviolet exposure or low dietary intake of vitamin D₃ in green turtles. Blood serum concentration of vitamin D₃ in wild turtles has apparently not been reported.