Abelsonite, a C11 nickel-porphyrin of the deoxophylloerythroetioporphyrin type, is shown to have methyl groups in the 2, 3, 7, 12, and 18 positions and ethyl groups in the 8 and 17 positions by high-resolution, high-field H nuclear magnetic resonance and nuclear Overhauser effect studies. Removal of the nickel by treatment with methanesulfonic acid permitted confirmation of the structure on the fee base porphyrin and demonstrated structural integrity under the conditions required for demetallation. The structure is best accounted for geochemically by the hypothesis that abelsonite is derived from a chlorophyll.