ABSTRACT.    Some studies have demonstrated that a few biological systems are affected by weak, extremely-low-frequency (ELF) electromagnetic fields (EMFs), lower than 10 mT. However, to date there is a scanty evidence of this effect on protozoa in the literature. The interest in experiments on protozoa consists in the fact that such tests can be considered as in vitro and in vivo. In the previous work, we showed the presence of propionylcholinesterase (PrChE) activity in single-cell amoebae of Dictyostelium discoideum. This enzyme activity was assumed to be involved in cell-cell and cell-environment interactions, as its inhibition affects cell aggregation and differentiation. In this study, we have exposed single-cell amoebae of D. discoideum to an ELF-EMF of about 300 μT, 50 Hz, from 1h up to 48h at 21 °C. The PrChE activity was inhibited significantly by 1h- and 3h-exposures to ELF-EMF, whereas it was similar to the control value after a 4h-exposure. A significant increase in this enzyme activity was found after 5h- and 24h-exposures, while a decrease in PrChE activity appeared in 48h-exposed sample. The increase detected in 24h-exposed cells returned to the control value, 24h after transferring the amoebae to standard conditions. The 3h- and 24h-exposed samples were processed for immunoblotting. In 3h-exposed sample the amount of protein recognized by the anti-mammalian butyrylcholinesterase antibody was similar to that of the control, whereas the increased quantity of the same protein was found in the 24h-exposed sample.