The growth of Escherichia coli B was investigated under homogenous 7 Tesla (T) magnetic field, and inhomogeneous 5.2∼6.1 T and 3.2∼6.7 T magnetic fields which are produced by a newly constructed superconducting magnet biosystem (SBS). These high magnetic field adversely affected on the growth of the bacterium in the early logarithmic growth phase. However, in the stationary phase, the cell number under a high magnetic field was about 2∼3 times higher than that of a control, indicating that the magnitude of the decrease in the cell number was reduced by the high magnetic field. The effect of the inhomogeneous magnetic field was much stronger than that of the homogeneous one. The high magnetic field was found to affect the cells of the bacterium differently, depending on the growth phases.