Langmuir-Blodgett (LB) films incorporating unsymmetrical merocyanine dyes; MC (X), (X=0, S, Se), have been fabricated. The J-aggregates of MC (S) and MC (Se) are characterized by a sharp J-band and resonance fluorescence. MC (0) forms a non-fluorescent aggregate having a broad absorption band. We have examined the formation of mixed J-aggregates in the MC (X)/MC (Y) assemblies (X, Y=O, S, Se). It has been found that a homogeneous aggregate of amalgamation type (HA-aggregate) is formed in the MC (S)/MC (Se) assembly. The mixed J-aggregate in the MC (0)/ MC (S) and the MC (0)/ MC (Se) assemblies are classified into a mosaic (M-) or a separate (S-) aggregate. Despite of similar molecular structure of MC (O) with both MC (S) and MC (Se), the growth of the MC (0) aggregate with a help of the MC (S) or the MC (Se) J-aggregate as a template may hinder the homogeneous mixing of dye molecules.