1. Sharp fluctuation of the intensity of the vertical component of the MF amounting to +/- 0.1 Oe changing the sign over each 3 min causes variability of both lysogenic and indicator strains of E. coli. This testifies to an extremely low threshold of their magnetic susceptibility and to biological importance of fluctuations of natural parameters of the geomagnetic field as an ecological factor of the environment. 2. A change in the intensity of the vertical component of the MF, not any higher than +/-0.1 Oe, inhibits phage production in the lysogenic system of E. coli K = 12 lambda and is also reflected in the morphological peculiarities of negative phage colonies as well as in the phage-susceptibility of the E. coli indicator strain.