Summary: Alcaligenes eutrophus strain H16 when grown on formate or oxalate as the sole source of carbon and energy had doubling times between 3.5 and 4.5 h. The respective molar growth yields (Y m) were 2.35 and 3.9. During growth on formate or oxalate both a soluble and a membrane-bound formate dehydrogenase were formed. The key enzymes of autotrophic CO2 fixation, ribulose-5-phosphate kinase and ribulosebisphosphate carboxylase, were formed during growth on formate but not on oxalate. Oxalate induced the synthesis of the enzymes of the glycerate pathway. Mutants impaired in autotrophic CO2 fixation but unaffected in the synthesis of the formate dehydrogenases lost their ability to grow on formate but not to grow on oxalate, giving further evidence that formate was assimilated via CO2.