Distribution of amino acids and related compounds in the muscle extract of seven species of right-eye flounder (spotted halibut, slime flounder, marbled sole, sand flounder, stone flounder, frog fleunder and bastard halibut) were studied. The effect of heat treatment on quantitative change in the composition of amino acids and related compounds in the extract of sand fleunder muscle was also investigated since the sand flounder has much Ex-nitrogen in the extract of the muscle. The content of crude protein and that of pure protein were in the range of $17.54{sim}19.99%$ and $15.63{sim}17.95%$, respectively. Among the extracts of the seven fish muscle, stone flounder showed the highest content of Ex-nitrogen(2.12%). In the muscle extracts of the seven fish taurine was abundantly contained $(29.4{sim}56.9%)$, and followed alanine $(6.6{sim}10.4%)$ and glycine $(1.6{sim}16.7%)$. The compositions of amino acids and related compounds were characterized by the existence of phosphoethanolamine, ${alpha}-aminoadipic;acid$, DL-allocystathionine, ethanolamine and ornithine. The experiments on amino acids and related compounds of the muscle extract of sand flounder with reference to heating time and temperature were resulted in that the amount of taurine, tyrosine, leucine and alanine were increased with the heating time at $100^{circ}C$, whereas that of lysine, histidine, ${alpha}-aminoadipic;acid$ and proline were decreased with prolonged heating time. When heating temperature was changed from $90^{circ}C$ to $130^{circ}C$ for 60 min, the contents of taurine, alanine and leucine were increased, while that of histidine, lysine and aspartic acid were decreased.