The surface morphology and molecular ordering of single-crystal sheets of photosensitive ionic dye molecules grown at the surface of a lipid monolayer are discussed. A photosensitive cyanine dye with SO-3 end groups was adsorbed from the water subphase onto monolayers of positively charged lipids : dimethyldioctadecyl-ammonium bromide (DODAB) or diacetylene-containing lipid dioctyl[(-bis-12-hydroxyethyl hexacosa-10,12-diynoate) ammonium bromide] (DAB). Lipid monolayers with adsorbed dye molecules were transferred onto silicon or glass substrates. Atomic force microscopy, X-ray reflectivity and optical fluorescence microscopy was used for characterization of the surface morphology of multiphase Langmuir-Blodgett films. Dye molecules adsorbed on a monolayer of the lipid DODAB formed perfect single crystals of nearly rectangular shapes with dimensions up to 100 mum across. The height of the crystals is equal to 1.5 +/- 0.2 nm which corresponds to the size of the molecules along the short in-plane axis. The dye crystals grown on a monolayer of DAB have a dendritic morphology. The height of the aggregates (3 +/- 1 nm) is more than twice that observed for the single-crystal sheet grown onto the DODAB monolayer. The observed differences of aggregation of dye molecules at the surface of monolayers composed of DODAB and DAB are discussed, taking into account the possibility of polymerization of the diacetylene-containing DAB lipid chains.