The interaction of a positively charged polyelectrolyte with the negatively charged 1,2-dipalmitoylphosphatidic acid monolayer at the air/water interface is studied by film balance, fluorescence microscopy, and surface X-ray diffraction measurements. It is shown that polymer binding leads to a local lattice expansion without insertion into the film at high lateral pressure (>20 mN/m). The film exhibits a rectangular structure with uniform chain tilt. The tilt angle decreases slightly with increasing lateral pressure. This contrasts with the film in the absence of polymer where a structure sequence oblique --> rectangular --> hexagonal was found. At low lateral pressures, the polyelectrolyte/lipid film shows coexistence of ordered and disordered regions with inhomogeneities gradually removed on pressure increase. The uniformity of the lipid/polyelectrolyte interaction is manifested in distinct and narrow diffraction peaks.