Bacteriochlorophyll b (Bchl b) is the principal green pigment of the photosynthetic bacterium Rhodopseudomonas viridis. The electron excitation spectrum of Bchl b is similar to but even more strongly red shifted than that of the bacteriochlorophyll a (Bchl a), 1. It enables this bacterium to use light down to 9800 cm-I (1020 nm), the least energetic light used by any known photosynthetic organism. The close structural relationship between Bchl a and b was established by Brockmann and Kleber, who converted both to 2 desvinyl-2-acetylpyromethylpheophorbide a and established Mg as the central metal and phytol as the esterifying alcohol. The most distinct difference between Bchl a and b is the easy conversion of the latter into products related to chlorophyll a (Chl a), which corresponds formally to oxidation of the macrocycle from the tetrahydro- to the dihydroporphyrin level. As cis chlorins are easily oxidized to porphyrins and show a red-shifted visible absorption spectrum compared to the corresponding trans epimers, Brockmann in 1970 formulated Bchl b as the 3,4-cis epimer of Bchl a (structure 2). At the end of the same year, Baumgarten proposed structure 3 for Bchl b, which formulates Bchl b as the A4,4a isomer of Chl a. The red shift in the electron excitation spectrum is explained by the CY$ unsaturation of the Bchl a chromophore, and the easy "oxidation" to the Chl a level as an isomerization. While the evidence of Baumgarten against 2 was convincing, it was not sufficient to establish 3. We can now present lH nmr, mass spectral, and chemical evidence that strongly supports 3 as the structure of Bchl b.