The uptake of succinate and malate had been compared in cultured cells and bacteroids of two species of slow-growing Rhizobium: R. japonicum (USDA I-110) and cowpea Rhizobium (USDA 3278). Cultured cells of both organisms actively accumulated both compounds, and uptake was abolished by KCN and 2,4-DNP, but not by arsenate. Kinetic studies using cultured cells showed that succinate competitively inhibited malate uptake, and vice versa, implying a common step in the uptake of these dicarboxylic acids. Uptake of both of these compounds was inhibited by osmotic shock and N-ethylmaleimide in cultured cells of both species. Purified bacteroids accumulated succinate in a process that was sensitive to 2,4-DNP and KCN, but at a rate significantly slower than for cultured cells. No detectable malate uptake was observed in purified symbiotic cells. Furthermore, succinate uptake was insensitive to osmotic shock in bacteroids of both strains. These results show that although bacteroids of both strains are competent in succinate uptake, significant differences exist in the expression and/or stability of dicarboxylate uptake systems between free-living and symbiotic cells.