The present work aimed at evaluating whether solubility and peptide profile of dietary protein can affect survival and growth performance of common carp during early stages of life. Water-insoluble casein and two forms of soluble casein (non-hydrolysed and hydrolysed) were used as protein sources at different proportions in the experimental diets for first-feeding carp larvae. When provided as the only protein source water-insoluble casein led to high mortality and depressed growth after 21 days. The partial or total substitution of water-insoluble casein by soluble non-hydrolysed casein effectively improved survival and enhanced growth of larvae. An optimum level of soluble non-hydrolysed protein was found at 25% of total protein. The addition of some hydrolysed casein (25% of soluble protein) further improved results. However, high dietary levels of hydrolysed casein led to negative effects on larval performance, which would be related with a surplus of di-/tripeptides or/ and amino acids. Protein solubility appears to have only a temporary effect on larval performance, probably related with the ontogeny of the digestive tract, since about 2 weeks after the onset of exogenous feeding larvae became able to use efficiently water-insoluble protein. Results also suggest the need for a balance in the peptide profile to optimise the utilization of dietary protein by fish larvae. (C) 2004 Elsevier B.V. All rights reserved.