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Methanogenic bacteria have been shown to require nickel for growth [I]. From these organisms a yellow nickel containing compound can be isolated [2-61, that has been designated factor FJso [7]. Its mass/m01 nickel was determined to be 1500 and egso to be near 23 000 cm-‘. 1 (mol Ni)-’[5, 6]. The structure and function of this compound is unknown. Incorporation studies with [14C] succinate as labelled precursor indicate that factor F,,,, may be a tetrapyrrole compound. 8 mol succinate/mol nickel were found to be incorporated into the factor, which is the amount predicted for a tetrapyrrole [4]. Tetrapyrrole biosynthesis proceeds via 6-aminolevulinic acid@-ALA) and porphobilinogen as intermediates. Here we show with Methanobacterium thermoautotrophicum that factor F,,, becomes labelled when the organism is grown in the presence of F-[4-14C] ALA. Dependent on the S-ALA concentration in the growth medium up to 8 mol s-ALA/m01 nickel were incorporated. Only tetrapyrroles are known to be synthesized from 8 mol &-ALA; therefore the incorporation data are taken as evidence that factor F J3,, has a nickel tetrapyrrole structure.