Diadema antillarum is ammonotelic in its excretion of nitrogenous waste products. It excretes approximately 60% of its total nonprotein nitrogen as ammonia and approximately 30% as amino acids. No urea, uric acid or other purine bases were found to be excreted. A progressive increase in tissue ammonia content in the intestine towards the rectum suggests that the hind gut has an excretory function.