The general features of accumulation of amino acids by marine invertebrates are outlined. The wide distribution of this ability is noted. Data from observations of this process for periods of several days are presented for the polychaete worm, Doruillea articulata. Measurements of the constituents in the pool of free amino acids of this same organism are presented together with data concerning the rate of appearance of radioactivity in the medium after permitting animals to accumulate amino acids. These data permit tentative estimates of rates at which amino acids are lost to the medium. These apparent “leakage” rates are low when compared with rates of uptake. The thermodynamic work necessary to move material against the concentration differences involved is calculated. The fraction of metabolic energy needed is small. The possible biological significance of uptake of small organic compounds is discussed. It is concluded that uptake of amino acids occurs sufficiently generally and at such rates that it should be included in any analysis of the pathways whereby material is acquired from the environment by marine organisms.