A feather abnormality was observed in chicks during bio-assay investigations with eleven essential amino acids. As dietary valine, leucine, isoleucine, glycine or phenylalanine and tyrosine levels decreased, a similar progressive feather abnormality became apparent. The outstanding feature of the feather abnormality was the concave structure of the feathers as they bent upward from the body. This structure gave the feathers a ragged appearance. The feather abnormality is described and illustrated. The percentage of dietary amino acids used were: valine, 0.40, 0.50, 0.60, and 0.70; leucine, 0.60, 0.68, 0.76, and 0.82; isoleucine, 0.32, 0.38, 0.44, and 0.50; glycine, 0.30, 0.35, 0.40, and 0.45; or phenylalanine and tyrosine, 0.65, 0.70, 0.75, and 0.80. Chicks fed the highest level of valine (0.50%), histidine (0.15%) or methionine to cystine ratio of 1:1 (0.30%) had normal feathers. In contrast, chicks fed the highest levels of either leucine, isoleucine, glycine or phenylalanine and tyrosine had abnormal feathers.