The roles of cyclic formaldehyde oxidation via 6-phosphogluconate dehydrogenase and linear oxidation via the tetrahydromethanopterin (H4MPT)-linked pathway were assessed in an obligate methylotroph, Methylobacillus flagellatus KT, by cloning, sequencing and mutating two chromosomal regions containing genes encoding enzymes specifically involved in these pathways: 6-phosphogluconate dehydrogenase, glucose-6-phosphate dehydrogenase and methenyl H4MPT cyclohydrolase (gndA, zwf and mch). No null mutants were obtained in gndA or zwf, implying that the cyclic oxidation of formaldehyde is required for C1 metabolism in this obligate methylotroph, probably as the main energy-generating pathway. In contrast, null mutants were generated in mch, indicating that the H4MPT-linked pathway is dispensable. These mutants showed enhanced sensitivity to formaldehyde, suggesting that this pathway plays a secondary physiological role in this methylotroph. This function is in contrast to Methylobacterium extorquens AM1, in which the H4MPT-linked pathway is essential.