ptoplasty is a remarkable form of symbiosis whereby photosynthetically functional algal chloroplasts are retained by the host organism. Certain Sacoglossan sea slugs form such associations and some species such as Elysia viridis (Montagu, 1804) can survive for months without access to their food algae. We report evidence for light-dependent assimilation of ammonium, nitrite, and urea (but not nitrate) in E. viridis with functional kleptoplasts. N-uptake was decreased in slugs with reduced chloroplast densities, and after exposure to inhibitors of glutamine synthetase and glutamate synthetase. Incorporation of 15N-labeled substrates was traced into individual hydrolyzable amino acids, with highest incorporation in glutamine-glutamate. N assimilation by kleptoplasts in multicellular hosts constitutes an additional mechanism which might contribute to the unique and long-lived functionality of these symbioses.