It is shown that a new procedure, based on genetic algorithms (GA's), can be used for direct determination of molecular constants, in particular rotational constants, from rovibronic spectra. This new approach only requires an estimate of the acceptable range of the parameters. The power of the method is demonstrated on the rotationally resolved fluorescence spectra of indole, indazole, benzimidazole, and 4-aminobenzonitril. A rigid asymmetric rotor Hamiltonian is used to calculate the theoretical spectra. The GA matches the generated spectra with an experimental spectrum with the use of a new method for spectra comparison. This spectra comparison function is able to deal with frequency shifts which are caused by (small) changes in the rotational constants and it yields better results in comparison with traditional spectra comparison methods, like RMS. In addition, the robustness of the method is tested.