Abstract: Growing in the medium containing KCl, NaNO3, (NH4)2SO4, the cells of the blue-green alga inflated in size and became spherical, the pigment region gathered at one side of the cell and the other side became transparent. The changes in cell structure were reversible and the effect of KCl could be offset by CaCl2. Growing in the nitrogen-free medium containing 0.1 mol KCl the trichomes became yellow and the N2 fixing activity decreased by 95%.