The chemical composition of Thalassia leaves is described. Of the leaf-dry matter, 40.6% consists of soluble nutrients: crude protein, lipids, soluble carbohydrates and ash. Crude protein content was 17% (R = 15.8-18.1%). Essential nutritive minerals were found in considerable quantities: calcium, 1.32% (R = 1.21-1.47%); phosphorus, .21% (R = .19-.23%); potassium, 3.0% (R = 2.47%-3.30%) and magnesium, 1.26% (R = 1.09%-1.38%). The cell wall or neutral-detergent fiber fraction was 59.4% (R = 46.1-64.6%), of which 19.1% consisted of hemicellulose. The lignin content varied from 22.9% (KMnO4) to 9.1% (H2SO4) depending on whether KMnO4 or H2SO4 was used as an oxidizing agent. The importance of Thalassia leaves as a food source for the seagrass community is described by five major food chains hereby proposed: the large herbivore, the fish herbivore, the gastropod herbivore, the urchin herbivore and the detrital food chains. The potential value of Thalassia as a food source for domestic animals is evaluated on the basis of comparison with forage crops.Se describe la composición química de las hojas de Thalassia. Del total de materia seca, 40.6% consiste de proteínas, lípidos, hidratos de carbono solubles y ceniza. El contenido en proteína bruta consiste de 17% (R = 15.8-18.1%). Los minerales nutritivos esenciales se encontraron en cantidades considerables: calcio, 1.32% (R = 1.21 - 1.47%); fósforo, .21% (R = .19-.23%); potasio, 3.0% (R = 2.47-3.30%) y magnesio, 1.26% (R = 1 .09-1.38%). Los componentes de la membrana celular fueron 59.4% (R = 46.1-64.6%), del cual 19.1% consiste de hemicelulosa. El valor porcentual de lignina varía, dependiendo de si KMnO4 ó H2SO4 se usa como agente oxidante: 22.9% (KMnO4) ó 9.1% (H2SO4). Se postulan en este trabajo cinco cadenas alimenticias básicas para describir la importancia de las hojas de Thalassia en la producción de organismos marinos: cadena alimenticia de herbívoros grandes, cadena alimenticia de peces herbívoros, cadena alimenticia de gasterópodos herbívoros y la cadena alimentica detrítica. Se evaluó el valor potencial de Thalassia como fuente de alimento para animales domésticos.