Naturally occurring geomagnetic storms have been shown to correlate with changes in organisms׳ biological processes. Changes in the geomagnetic field during a geomagnetic storm are complex and contain both slow changes of the geomagnetic field with frequencies of up to 0.001Hz, and various geomagnetic pulsations observed in general to be within the range of 0.001–5Hz. Little is known about what frequency constituent of geomagnetic storms has the strongest effect on living organisms. This paper uses an experimental approach to demonstrate that organisms from different taxa principally respond to slow changes of the geomagnetic field corresponding with the main phase and the initial period of the recovery phase of a geomagnetic storm. Pc1 type pulsations, which are commonly regarded as biologically effective elements of geomagnetic disturbances, did not affect controlled parameters in our experiments. This paper may serve as a starting point for a thorough inquiry into the influence of slow fluctuations of the geomagnetic field on organisms.