The contribution of epiphytes associated with physical substrates to the nutritional requirements of post-larval shrimp, Penaeus esculentus Haswell, was determined in high-density rearing systems (3000, 6000 and 11 000 m-3). Stable isotope signatures of epiphytes on polyethylene mesh substrate, AquaMats頡nd tank walls were compared with shrimp signatures. Two methods were used: the determination of carbon and nitrogen natural abundance ratios; and 15N-nitrogen enrichment ratios after the addition of 15N-ammonium to tanks. Using the natural abundance technique and a simple mixing model, epiphytes were found to contribute substantially to the carbon requirements of post-larval shrimp (39-53%). This was despite the addition of formulated feed at satiation levels. There was no indication of a reduced contribution of carbon from epiphytes to shrimp nutrition at higher shrimp densities. The lack of a difference in the 15N/14N ratios of the two food sources meant that mixing models could not be used to calculate the contribution of nitrogen from epiphytes vs. artificial feed to shrimp nutrition. Using the 15N-nitrogenenrichment method, the amount of nitrogen contributed by epiphytes to shrimp nutrition over 24 h could be determined. This method showed that nitrogen from epiphytes was assimilated by shrimp. 15N-enrichment methods provided a more accurate alternative to natural abundance techniques, particularly when the stable isotope signals ofthe food sources are similar. This experiment hasshown the benefits in providing substrates for P.esculentus in high-density rearing systems to provide an additional food source for shrimp.