Histological examination of long bones from wild American Alligators (Alligator mississippiensis) reveals weakly defined growth zones and extensive fibrolamellar bone formation in both healthy and stressed adult individuals. The discovery of this tissue in wild adults of variable health status challenges three current assumptions concerning fibrolamellar bone in crocodylians: first, that it can only be formed in juveniles; second, that it occurs only in captive animals; and third, that it occurs only in individuals experiencing optimal physiological conditions. This highlights the need for further detailed study of bone microstructural diversity among modern crocodylians. Moreover, these results have potentially far-reaching implications for studies of fossil archosaurs, which often draw on extant crocodylian models, and for the phylogenetic significance of fibrolamellar bone within Archosauria as a whole.