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The approximately linear relationship between ionospheric critical frequencies and sunspot numbers is well known. Examination of ionospheric data from the few locations at which ionosphere observations have been made for substantial parts of two sunspot cycles shows appreciable differences in the slope of the line of correlation between critical frequencies and sunspot numbers for different cycles. This is an additional indication that the Zurich sunspot number is only an approximate index of the solar activity responsible for ionospheric ionization. The differences between cycles should be taken into account in the preparation of ionospheric radio frequency propagation predictions.