A new type of dialysis culture system was used to cocultivate different bacterial species under oligotrophic conditions. In a mineral medium with no added carbon source, the tested Rhizobium leguminosarum bv. phaseoli strains outgrew an Arthrobacter strain, but not an Enterobacter strain. The Rhizobium strains had generation times of approximately 4.3 h, Enterobacter agglomerans 1.5 h, and Arthrobacter globiformis more than 30 h. After adding glutamate to the system to a final concentration of either 2 or 10-mu-M, we found significant (P < 0.05) differences in growth rates between Rhizobium leguminosarum bv. phaseoli strains. Strains Ciat 899 and Kim5s showed maximal growth at the 10-mu-M glutamate concentration with generation times of 2.8 and 3.1 h, respectively. In contrast, strains Ciat 895 and CE3 reacted with increased generation times of 3.8 and 4.3 h. However, when immunofluorescence techniques were used to follow populations of the strains in an acid and a neutral tropical soil, no significant strain differences in decline were observed. Thus, strain survival in unplanted soils was not related to growth rates under oligotrophic conditions. In contrast to this, the ability to grow more or less rapidly under carbon-limited conditions was positively correlated with the known competitiveness of Rhizobium leguminosarum bv. phaseoli strains in legume host nodulation.