Membrane protein patterns in SDS-PAGE clearly characterized Azospirillum amazonense, A. brasilense, A. lipoferum, and Herbaspirillum seropedicae. Each species showed also a typical major band with approximate molecular weights (AMWs) of 44, 39.5, 35.5, and 37.5 Kdaltons for the four species respectively. Characteristic lighter bands with AMWs of 63.5 and 19 Kdaltons were shown by A. amazonense. Similar bands of 54, 24 and 24.5 Kdaltons were typical of A. brazilense, A. lipoferum and H. seropedicae, respectively. The three Azospirillum species showed four identical proteins in common, out of the six main proteins presented in SDS-PAGE; H. seropedicae had only two major proteins with one of them common to all three Azospirillum species. The data strongly favors the maintenance of Azospirillum and Herbaspirillum as valid generic taxa as well as supports the validity of the three Azospirillum species.