Conformational behaviors of three synthetic copolypeptides [GA (L-Glu:L-Ala, 6:4), GAT1 (L-Glu:L-Ala:L-Tyr, 6:3:1), and GAT2 (L-Glu:L-Ala:L-Tyr, 1:1:1)] in the interfacial films were studied. In the spread monolayers at the air/water interface, GA takes the alpha-helix conformation, regardless of the surface pressures. GAT1 and GAT2 containing tyrosine residue exhibit the mixed conformations of alpha-helix and beta-sheet, in which the ratio of alpha-helix to beta-sheet depends upon the surface pressure. GAT1 with less content of tyrosine residue takes an alpha-helix-rich conformation, while GAT2 with more tyrosine residue has a beta-sheet-predominant structure at lower surface pressures. A transformation from the a-helix to the beta-sheet occurs with increasing surface pressures. On the other hand, these copolypeptides were incorporated from the aqueous subphase into monolayers of lipids such as stearyl amine. The structure of the polypeptides in the adsorbed interfacial films is substantially similar to those of the spread monolayers at lower surface pressures, but there are some disordered structures such as a random coil in the adsorbed state.