In Sinorhizobium meliloti strain Rm1021, ExpR+ mutation results in the overproduction of EPS II, which is required for efficient invasion of root nodules on the host plant alfalfa. When rhizobia were grown in LB/MC medium for 36-hour then placed at room temperature, most of ExpR+ mutant (Rm8530) cells aggregated at the bottom of the tubes, but ExpR+ sinR double mutant Rm11528 and wild type Rm1021 did not. The ExpR+ mutant was also found to swim slower than Rm1021 on swarming plates, but the ExpR+ sinR- mutant showed almost the same as wild type. The average diameter of swarming plaques for Rm8530, Rm11528 and Rm1021 was 13, 16 and 16 mm respectively, when bacteria were incubated at 28 degrees C for two days. After four days, the plaques enlarged to 17, 22 and 20 mm, respectively. These results indicate that ExpR+ mutation causes a serious defection of motility in this condition. By flagella staining with silver nitrate method, it was noted that all three strains had flagella. It suggests that the swimming behavior of Rm8530 may not result from the change in components and structures of flagella. Based on DNA microarray data, Hoang speculated that the Sin system seemed to regulate a multitude of genes in S. meliloti, including genes that participate in succinoglycan production, motility, and chemotaxis, as well as other cellular processes (Hoang et al. 2004). And most of the regulation by the Sin system was dependent on the presence of the ExpR regulator. Accordingly, the expression of the flaA, cheY1 and motC operon was determined using promoter: lacZ fusion in different genetic backgrounds. The results showed two fold lower of motC expression activity in ExpR+ mutant strain Rm8530 than in wild-type strain Rm1021 at early exponential phase and this decrease was hold back by further mutation of sinR. However, the beta-galactosidase activity of motC-lacZ fusion was almost the same in three strains at later exponential phase. These results suggest that ExpR may repress the expression of motC operon in a low cell density, but this repression can be deprived in a higher cell density. It may be a good explanation to the motility of Rm8530 on swarming plates, since it is lower cell density of bacteria on the swarming plates. Therefore, it may be concluded that ExpR both involved in the regulation of EPS II production and motility in S. meliloti.