The application of two bacteria as biosystems for determining the growth effects of a high magnetic field was studied. Multiplication of Bacillus subtilis and of Enterobacter aerogenes was observed during exposure to a magnetic field of 6100 gauss. Cell populations were studied at 2 hr intervals, and the results were considered statistically. No effect of the high magnetic field was seen with B. subtilis. On the contrary, significant inhibition of E. aerogenes occurred at 5 hr. This effect disappeared at 11 hr, since control and exposed cultures became indistinguishable.