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The collisional frequency of free electrons with neutral air molecules and atoms in the earth's atmosphere is calculated on the basis of contemporary ideas concerning temperature, pressure, composition, and dissociation of atmospheric gases. Expressions are developed for these calculations. Since contemporary ideas on the state of the upper atmosphere are markedly different from those held ten years ago, this new determination of collisional frequency yields values different from those found earlier. Two sets of values are worked out, one for night and the other for day conditions. It is believed that these two cases represent maximum and minimum values. The effective molecular diameter is discussed. It is pointed out that more exact knowledge of the collisional frequency in the ionospheric regions is important in connection with theories of radio wave propagation.