AbstractThe Sirenia along with the elephants and hyraxes form the Super‐Order Paenungulata. They are the only fully aquatic mammalian herbivores and are represented by three species of manatees (Trichechidae), the dugong and the now extinct, Steller's sea cow (Dugongidae). Sirenians are non‐ruminant herbivores that have specialized hind‐gut fermentation in a large paired caecum at the juncture of the large and small intestines. The stomach is relatively small and is characterized by a unique cardiac gland which contains most of the digestive‐enzyme secreting cells. Little is known of the digestive physiology of sirenians. The metabolic rate of the manatee is very low in comparison to other mammals and may even represent the lowest weight‐specific metabolic rate for any mammal known. Metabolic rates forDugonghave not yet been determined. Foods and feeding habits of these species are reviewed both qualitatively and quantitatively. Manatees consume approximately 896 of their total body weight in aquatic plants daily, whereas the same value for the dugong is about 14%. Digestibilities of aquatic plants vary from 45 to 70% for manatees and a singlein vitromeasurement forDugongwas 83% for sea‐grass. The chemical composition of various plants consumed by wild and captive sirenians are presented and their value in the nutritional ecoloev of the different species i