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Changes in the free and peptide-bound amino acids during the 3rd and 4th larval instars and the pupal stage of Aedes aegypti (L.) (Diptera: Culicidae) were studied. Thirty-six ninhydrin-positive compounds were separated on chromatograms of deproteinized insect extracts. Glutamic acid, alanine, tyrosine, and arginine consistently occurred in high concentration among the free amino acids, whereas among the peptide-bound amino acids, glutamic and aspartic acids and glycine were the principal components. In spite of various fluctuations, no definite developmental patterns emerged from the study, and no specific correlation of amino acid changes with development was possible. The possible significance of several of the changes are discussed. Several nonprotein amino derivatives were also present. The occurrence of citrulline and urea during only the larval stage suggested that urea may be an important excretory product during this stage.