A stable radical exists in mixed Langmuir-Blodgett (LB) films of a merocyanine dye, having benzothiazole as a donor nucleus, and arachidic acid. The wavefunction of the radicals has been studied by using the N-14 hyperfine coupling observed in films of pristine dye and the C-13 coupling observed in films using a C-13-enriched type. The magnitude and symmetry of N-14 and C-13 hyperfine tensors, obtained from the spectra, show firstly that the radical wave function extends over the dye-chromophore part and, secondly, that the pn orbital axes of nitrogen in benzothiazole and the carbon of the central methine chain are nearly parallel. The latter fact excludes the possibility of a spyropiran form as the origin of the radical. These facts show that the generation of the stable radical is a phenomenon associated with the dye molecules in LB films and detail of the wavefunction possibly probes the electronic states of the dye molecules in aggregates.