A comparative study on the isolated chlorosomes from Chloroflexus aurantiacus, a green filamentous photosynthetic bacterium and Chlorobium tepidum, a green sulfur photosynthetic bacterium, was done by ODMR (optically detected magnetic resonance). Correlation between the results obtained by fluorescence and absorption detection is shown to be a source of information about the functional interactions among pigments. Analogies and differences are pointed out between the light-harvesting systems of the two species. Triplet states are easily detected in both bacteria at 1.8 K under steady-state illumination and are assigned to BChl c, BChl a, and carotenoid molecules. Carotenoids are found to be able to quench BChl a triplet states, but no evidence of BChl c triplet states quenching by this triplet−triplet transfer mechanism is found in both systems. Then from the data it appears that some carotenoids are in close contact with BChl a molecules. The relevance of this finding to the localization of carotenoids in the chlorosomes is discussed. In Cb. tepidum three different pools of BChl c oligomers connected to BChl a were found by detection of their triplet state, while only one pool of BChl c was evidenced in Cf. aurantiacus. The latter appears to be unconnected, at least at 1.8 K, to BChl a. On the other hand, heterogeneity in the BChl a triplet population was detected in Cf. aurantiacus. Even though the two bacteria show common features in the way the light excitation induces triplet formation at low temperature, the detected triplet states show spectroscopic properties that strongly depend on the system. The results clearly indicate that differences in pigment organization exist both in the core and in the baseplate of the chlorosomes from the two different bacteria.