The free and protein amino acids of Toxascaris leonina occurring in the red fox (Vulpes vulpes) were tentatively identified by the method of descending two-dimensional paper chromatography. Lysine, histidine, arginine, glycine, serine, threonine, glutamic acid, alanine, proline, tyrosine, methionine, valine, phenylalanine, and leucine-isoleucine were found in the free amino acid fraction. The protein amino acid fraction revealed cystine, lysine, aspartic acid, histidine, arginine, glycine, serine, threonine, glutamic acid, alanine, proline, tyrosine, methionine, valine, phenylalanine and leucine-isoleucine. The presence of tryptophane was not determined as alkaline hydrolysates were not prepared.