Green hydra, which contain intracellular Chlorella symbionts, rapidly took up ammonium added to their medium. Aposymbiotic hydra artificially rid of their symbionts did not take up ammonium but released it. Uptake was concentration dependent, showing kinetics typical of diffusion rather than active transport, and was prevented by treatment with the glutamine synthetase inhibitor methionine sulphoximine. Uptake by freshly isolated symbiotic algae was lower than in the intact symbiosis, and treatment with the photosynthetic inhibitor DCMU reduced ammonium uptake to a greater extent in the intact symbiosis than in freshly isolated symbionts. This suggests that uptake was at least partly caused by the activity of animal glutamine synthetase. Uptake is discussed with reference to control of symbiont growth and division, and the nitrogen budget of the symbiosis.