The interaction of a newly synthesized cationic intercalator amphiphile C(18)AO into double-stranded DNAs at the air-water interface was investigated. Polyion complex monolayers consisting of DNAs and cationic amphiphiles were prepared, Pressure-area isotherms. fluorescence measurements, Fourier transform infrared-reflection-absorption spectroscopy and quartz crystal microbalance measurements suggest that C(18)AO molecules can be intercalated into base-pairs of DNAs to form densely packed two-dimensional DNA arrays at the air-water interface.