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Triplet state electron spin resonance (ESR) spectroscopy has been used to study triplet-triplet energy transfer in B800–850 light-harvesting complexes and carotenoporphyrin molecules. The B800–850 complexes were isolated from the photosynthetic bacteria Rhodobacter sphaeroides GA, aerobically and anaerobically grown Rb. sphaeroides wild type and Rhodopseudomonas acidophila 7750. Free-base and zinc-substituted carotenoporphyrins featuring various linkage structures and different orientations of the pigments were studied. The carotenoporphyrins which contain a short bridging link display ESR spectra which resemble those of the B800–850 complexes. The results indicate a close spatial proximity between the bacteriochlorophyll and carotenoid pigments in the B800–850 complexes which leads to efficient triplet-triplet energy transfer. Computer simulations of the observed ESR spectral line-shapes yielded values for the zero-field splittings which can be understood in terms of the varying extents of π-electron conjugation in the carotenoids. The rate constants for population and decay of the observed triplet states were also obtained from the computer simulations. All of the carotenoid triplet states exhibit similar ESR spectral lineshapes and are characterized by the spin polarization pattern eae aea . The molecular basis for the spectral uniformity may be explained by triplet energy transfer according to the exchange mechanism and conformational changes of the carotenoid which lead to triplet spin relaxation.