[en] The high-resolution ¹³C NMR spectrum of bacteriochlorophyll a biosynthesized from [1-¹³C,1,1,4-¹⁸O₃]-5-aminolevulinic acid by growing cells of Rhodopseudomonas sphaeroides has shown both the C-17³ and C-13³ resonances consist of three additional components upfield shifted from the -¹⁶O- ¹³C double bond ¹⁶O resonance. By comparison with the ¹³C NMR spectrum obtained for phytyl acetate containing ¹³C and ¹⁸O selectively in the ester linkage, these components have been identified as the bridge (-¹⁸O- ¹³C double bond ¹⁶O), non bridge (-¹⁶O-¹³C double bond ¹⁸O), and dual-labeled (-¹⁸O-¹³C double bond ¹⁸O) isotopomers, These results have been interpreted to suggest that both the ester bonds of bacteriochlorophyll a are produced by a carboxy-alklyl transfer process