We investigated the effect of chain length of dietary medium-chain fatty acids (MCFA) on growth performance and fatty acid composition of first-feeding carp larvae. In a first trial, five semi-purified isolipidic (23-24 g/100 g of dry matter) diets were formulated to contain either 10 g/100 g triolein (control diet) or 5 g/100 g triolein and 5 g/100 g medium-chain triacylglycerols (MCT) supplied as tricaproin, tricaprylin, tricaprin or trilaurin. After 21 d, survival and growth rates were significantly greater in larvae fed diets containing triolein, tricaproin, tricaprin and trilaurin (final survival: 92 +/- 7% and mean larval weight: 42 +/- 15 mg) than in larvae fed tricaprylin (final survival: 56 +/- 12% and mean larval weight: 15 +/- 1 mg). The recovered levels of the fed MCFA in larval total lipids were respectively 0, 1.3, 7.3 and 8.1 g/100 g of total fatty acids. In a second trial, two isolipidic (18 g/100 g) diets containing 10 g/100 g triolein or tricaprylin were tested. High amounts of capric acid (up to 25 g/100 g of total fatty acids) were found in neutral lipids of carp larvae fed tricaprylin for 11 d, suggesting an unusual elongation of caprylic acid. This study underlines the peculiarity of tricaprylin among other MCT which seem well utilized up to 20-30 g/100 g of total dietary fatty acids. The exception of tricaprylin raises the question of the metabolic pathways followed by this MCT, especially for the suggested direct elongation of caprylic acid into capric acid.