Metamorphosing leptocephalous larvae of the bonefish (Albula sp.) were analyzed for total ninhydrinpositive substances (NPS) and free amino acids. Total NPS content showed little change during metamorphosis. The average NPS value (±S.E.) for 16 larvae was 1.8 (±0.1) mg×larva(-1), which represents approximately 4% and 8% of the total dry weight of early and advanced larvae, respectively. Taurine was the most abundant free amino acid in whole-larva extracts, accounting for 36% and 59% of the total by weight in early and advanced larvae, respectively. The essential amino acids, leucine, isoleucine, phenylalanine, histidine, valine, methionine, lysine and arginine, accounted for about half (47%) of the total in early larvae but were reduced to about 23% of the total in advanced larvae. All of the component essential amino acids decreased during metamorphosis, but the greatest effect was seen with the first five. The remaining non-essential amino acids comprised less than 20% of the total in early larvae and, although the overall value changed little during metamorphosis, certain components such as glycine and glutamic acid showed large increases whereas others such as tyrosine and serine were reduced. Increases in amino acid content after acid hydrolysis of whole-larva extracts indicated that trichloroacetic acid-soluble, low molecular weight peptides were present in both early and advanced leptocephali.