Leuconostoc mesenteroides B-1299 dextrans are separated into Two kinds : fraction L, which is precipitated by an ethanol concentration of 38%, and fraction S, which is precipitated at an ethanol concentration of 40%. Fraction S dextran contained 35% of alpha-1,2 branch linkages, and fraction L contained 27% alpha-1,2 branch linkage with 1% alpha-1,3 branch linkages. We have isolated mutants constitutive for dextransucrase from L, mesenteroides NRRL B-1299 using ethyl methane sulfonate. The mutants produced extracellular as well as cell-associated dextransucrases on glucose media with higher activities (2.5-4.5 times) than what the parental strain produced on sucrose. Based on Penicillium endo-dextranase hydrolysis, mutant B-1299C dextransucrases produced slightly different dextrans when they were elaborated on a glucose medium and on a sucrose medium. Mutant B-1299CA dextransucrase elaborated on a glucose medium and on a sucrose medium synthesized the same dextran, although the dextran was different from those of other mutants and the parental strain. Mutant B-1299CB dextransucrase, elaborated on a glucose medium and on a sucrose medium, formed different dextrans. Differences in water solubility, susceptibility to endo-dextranase hydrolysis, and the physical appearance of the ethanol precipitated dextrans elaborated by different mutants grown on glucose media and sucrose media were found. All mutant dextransucrases elaborated on a glucose medium bound to Sephadex G-200. After activity staining of nondenaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis activity bands, 184 and 240 Kd for each enzyme preparation, although each dextransucrase formed different dextran(s).