The electronic structure of the circular aggregate of 18 bacteriochlorophyll a (BChl a) molecules responsible for the B850 absorption band of the light-harvesting 2 (LH2) complex of the photosynthetic purple bacterium Rhodopseudomonas acidophila has been studied by measuring fluorescence-excitation spectra of individual complexes at 1.2 K. The spectra reveal several well-resolved bands that are obscured in the single, broad B850 band observed in conventional absorption measurements on bulk samples. They are interpreted consistently in terms of the exciton model for the circular aggregate of BChl a molecules. From the energy separation between the different exciton transitions a reliable value of the intermolecular interaction is obtained. The spectra of the individual complexes allow for a distinction between the intra- and the intercomplex disorder. In addition to the random disorder, a regular modulation of the interaction has to be assumed to account for all the features of the observed spectra. This modulation has a C2 symmetry, which strongly suggests a structural deformation of the ring into an ellipse.