The aldehyde dehydrogenase activity in 21 species or strains of the genus Pseudomonas was determined in cell-free extracts prepared by a standard procedure. Organisms grown in a salts – yeast extract – tryptone – ethanol medium all possessed an enzyme active on glycolaldehyde, using nicotinamide adenine dinucleotide (NAD+) as electron acceptor, which was activated by potassium ion. None of the organisms possessed an aldehyde dehydrogenase activated by phosphate or arsenate, regardless of the degree of use of ethanol from the medium.