Measurements with linearly polarized exciting and fluorescent light show that the excimerlike fluorescence of pyrenebutyric acid in alkaline solution is polarized. The degree of polarization r depends on temperature and, very pronounced, on the wavelength of excitation. This occurs in the spectral region where the excitation spectrum of the excimer differs from the corresponding monomer spectrum. The value of r, after correction of rotational depolarization, is 0.36 and indicates that single transition moments of absorption and emission coincide nearly in their direction. These results, together with the fluorescence time decay laws, strongly support the conclusion that, at least in an hydrophilic surrounding, not excimers but ground-state stable dimers of pyrene are formed.