The nutritional contribution from co-fed Artemia nauplii and inert diets to growth in mysis and early postlarval shrimp Litopenaeus vannamei was assessed by analyzing the carbon stable isotope ratios (δ 13 C) in diets and shrimp tissue. Artemia nauplii and inert diets showed significantly different δ 13 C values but similar carbon contents and were supplied as single diets and also in three co-feeding regimes in which 25, 50 and 75% of the Artemia was replaced by inert diet on a dry weight basis, so that all feeding regimes provided similar amounts of dietary carbon. Shrimp δ 13 C values were significantly influenced by the different feeding regimes and reached isotopic equilibrium with their respective diets as soon as 5 days. Survival was significantly higher in co-fed animals than in those fed either Artemia or inert diet alone. There was no significant difference in growth between shrimp fed on Artemia only and those co-fed Artemia with inert diet, although the variability was high. Growth and survival were very low in shrimp fed only inert diet. Results from an isotope mixing model suggest that observed nutrient contributions from Artemia nauplii were significantly higher than expected contributions indicated by proportions established in the co-feeding regimes. Nutrient contributions to growth in the dietary regime providing equal carbon amounts of each diet type ranged from 77 to 86% for Artemia and from 13 to 24% for inert diet. Shrimp fed the 25% Artemia replacement regime exhibited a significantly higher retention of dietary carbon from the inert diet than those fed inert diet alone. This may have resulted from greater ingestion and/or assimilation of the inert diet in the presence of Artemia, combined with the higher growth rate in the co-feeding treatment. The results demonstrate the effectiveness of up to 50% replacement of Artemia with inert diet for L. vannamei mysis and early postlarval stages, indicating also that the inert diet may provide specific nutrients that promote survival, while digestibility may limit its contribution to tissue growth. The 3 influence of maternal and dietary sources on δ 13 C values in earlier larval stages was also described.