Temperature affects the expression of the three different nitrogenases in Azotobacter vinelandii. Molybdenum repressed the vnfH and anfH operons relatively more at 30°C than at 20°C; at 14°C molybdenum did not repress these genes at all. Similarly, V repressed the anf operon at 30°C but not at 20 or 14°C. Mo was poorly transported into cells grown at the lower temperatures. A. vinelandii thus has the potential to synthesize any of the three nitrogenases at 14 to 20°C regardless of the presence of Mo or V.