Observations of attenuation of cosmic radio noise, made at 18.3 Mc/s during the period November 1958 to December 1960 at Makapuu Point, Oahu, Hawaii, have been subjected to a statistical study, and the semidiurnal lunar tidal variation has been isolated. The amplitude was found to be 0.17 decibel, the maximum occurring at 11.6 lunar hours after lunar transit. It is argued that the tidal variation observed is primarily associated with the F2 layer of the ionosphere by citing previous studies of tidal variations in the E, Es, F1, and F2 layers and comparing the phases of the variations found by other workers with the phase isolated in this study. The phase found agrees with that found for tidal variations in the critical frequency of the F2 layer but not with variations of critical frequency in the other layers. On the other hand, it is argued that the D layer plays a minor role, owing to its absence at night and predominant role of the F2 layer in determining the attenuation of cosmic noise when the critical frequency exceeds 10 Mc/s.