Monolayer formation of hydrophobic a-helical peptides, X-(Ala-Aib)(8)-Y (X = Boc-, HOOCCH2CH2CO-, biotinyl, biotinyl-(Sar)(3)-; Y = OMe, OBzl, OH), at the air/water interface was studied by the fluorescence microscopic method. Some peptides showed a mound in the pi-A isotherm. When the monolayer containing a small amount of FITC-labeled peptide was held at the surface pressure corresponding to the top of the mound, bright and dark domains were observed by fluorescence microscopy. Domain formation was also observed by the addition of a cationic dye (DiIC(1)) into the subphase underneath the peptide monolayer. The mound in the pi-A isotherm is, therefore, ascribed to the phase transition from an expanded state to a condensed state. Two different shapes (leaflet and needle) of solid domains were observed and discussed in terms of different orientations of the peptides in the monolayer.