Sisomicin fermentations were carried out in stirred and air lift fermenters, and various concentrations of MgSO4 were supplemented to improve antibiotic yield. The highest antibiotic yield was obtained at 80 mM in an air lift fermenter due to effective liberation of intracellular sisomicin by MgSO4. The presence of high concentrations of MgSO4, along with shear stress due to strong agitation in the stirred fermenter, resulted in low values of cell activity, rheological parameter(n) and cell viability in the stirred fermenter, compared with the air lift fermenter. The better physiological states of cells grown in the air lift fermenter resulted in improved antibiotic yields.