All organisms surveyed among a range of methylotrophic bacteria growing on both C, and multi-carbon substrates contained a dye-linked formaldehyde dehydrogenase. The activity of this enzyme was very low in bacteria grown on C, compounds when compared to the activities of other C,-specific enzymes and was not induced during growth on C, compounds. The enzyme was partially purified (1 1-fold) from methanol-and ethanolgrown Hyphomicrobium X. Though not homogeneous, it did not contain methanol dehydrogenase, formate dehydrogenase or cytochrome c. Methylphenazonium methylsulphate (PMS), cytochrome c, Wurster's Blue, dichlorophenol-indophenol (DCPIP) and methylphenazonium ethosulphate (PES) could act as the primary electron acceptors in the assay system with apparent K, values for the first three substances of 0.69, 128 and 3.07 ,!AM, respectively. The activity of the partially purified enzyme could be maintained over 3 months at -20 "C in the presence of ethanol (lo%, v/v). The molecular weight of the enzyme was 83 500 k 2500. It was active from pH 6.5 to 9-0 with maximum activity at pH 7.2 using cytochrome c as the electron acceptor and at pH 7.6 when PMS and DCPIP were used. Several different aldehydes could act as substrates; the apparent Km values for formaldehyde, acetaldehyde, propionaldehyde and heptaldehyde were 2860, 270, 13 and 2.5 ,UM, respectively. Methanol and ethanol were not substrates. The dye-linked formaldehyde dehydrogenase is probably a general aldehyde dehydrogenase not directly involved in the dissimilation of C1 compounds.