Digestibility and passage time were measured in Gopherus polyphemus fed leaves of the legume Aeschynomene americana. Intake was held constant relative to body mass. Mean apparent digestive efficiencies were 68% of organic mattes, 61% of energy, 73% of cell walls, and 71% of nitrogen. Mean passage time was 13 d. Body mass, within the range of 3.3-7.9 kg, had no effect on digestibilities or passage time. Potassium permanganate lignin was not an acceptable indigestible dietary marker, in vitro indigestible cell walls was an accurate indigestible marker. Gopherus polyphemus is as capable of maintaining an efficient gut cellulolytic microflora and degrading the fiber fraction of its diet as are tropical reptiles. ©1987 American Society of Ichthyologists and Herpetologists. Accessible at BioOne database http://www.bioone.org/