Semiempirical AM1 studies on phycocyanobilin, the most abundant chromophore in the light-harvesting phycobiliproteins and a suitable model of the phytochrome chromophore, have been carried out. For the all-Z isomer, the structures and energies of all possible conformers, which arise from rotation about the single bonds of the exocyclic methine bridges, were calculated. In addition, the rotational barriers between the different conformers were deduced from the computed energy hypersurfaces. The cyclic helical SSS conformation is the most stable structure. Other minimum-energy structures with well-separated local minima on the energy hypersurface are the partially extended ASS and SAS conformations, although the conformational situation is charac-terized by a high degree of kinetic flexibility. The energy of the different conformers is mainly governed by intramolecular hydrogen bonding and steric repulsion of the substituents at the tetrapyrrolic backbone. It is very likely that the energetically most favorable conformers, SSS, ASS, and SAS, correspond with the three ground-state conformers of phycocyanobilin detected in previous time-resolved absorption studies.