In order to examine the relationship between the amino acid assemblage in the sediments and the depositional environment of the basin, the amino acids in 18 marine muddy sediments of the Kusanagi Formation (middle Miocene) of the Shinjo district, Yamagata Prefecture, were analysed. The analysis of amino acids was carried out by using the automatic amino acid analyser -JEOL model 5 AH type-. The results are summarized as follows; (1) a non-protein amino acids-basic amino acids-acidic amino acids triangular diagram (Sasaki, 1973 a) which had been proposed from the data of the amino acid assemblage in the sediments from the Chuetsu district, Niigata Prefecture, may also be used to estimate the depositional environment of the basin in the case of the study area: (2) hard black colored mudstone member of the upper part of the Kusanagi Formation, which shows a remarkable change in the lithology to marginal facies towards the northeast of the basin and is distinguished from the middle and lower parts by the forarniniferal assemblage, is characterized by the higher relative content of non-protein amino acids compared with the middle and lower parts of this formation. This upper part is also characterized by the high concentration of polycyclic aromatic hydrocarbons (Taguchi et al., 1970). It is concluded that non-protein amino acids in the sediments from the study area may have been derived directly from terrestrial organic mater.