The electrophoretic mobility of Rhizobium trifolii strain TA1 was relatively constant during the early and late phases of growth, but exhibited a temporary sharp increase in the late logarithmic-early stationary phase. The electrophoretic behavior of slow-growing species of Rhizobium was characterized by zero mobility at pH 2.0, and a constant negative mobility between pH 4.0 and 10.7. In contrast, all fast-growing strains, except R. trifolii TA1, exhibited slight positive mobility at pH 2.0, a constant negative mobil-ity between pH 4.0 and 9.2, and a marked increase in mobility between pH 9.2 and 10.7. These results suggest that the surfaces of the slow-growing bacteria and strain TA1 contain only acidic (carboxyl) groups, whereas those of the fast-growing strains contain a predominance of acid groups along with some basic (amino) groups. The electrophoretic behavior of a number of variants of R. trifolii strains SU297 and SU298 apparently is related to antigenic structure, but not to nitrogen-fixing ability. The relationship of surface charge density to the poor expression of lysogeny in one variant (SU298D) is discussed. Variants of R. trifolii strain NA34 differing in their infective ability exhibited identical electrophoretic mobilities.