A field study on morphology and natural history of the Yucatán spiny-tailed iguana Ctenosaura defensor. During a four-week excursion to the Yucatán-Peninsula, the author observed and examined 19 specimens of Ctenosaura defensor in the field (near Telchaquillo and Tetiz). This terrestrial species inhabits rocky plains of the Mexican state of Yucatán. It hides in the holes of marine mussels in lime-stone rocks. The examination of fecal samples of free-living C. defensor indicates that it feeds predominantly on leaves. Native people report that the Yucatán spiny-tailed iguana produces clutches with only 2-3 eggs.