The annual variations of the amplitude, period, and probability of existence of foF2 quasi-2-day oscillations at middle latitudes in both northern and southern hemispheres are investigated. The foF2 hourly data for the period 1977--1982 for stations Kiev (50.5 ¿N, 30.5 ¿E) and Wakkanai (45.39 ¿N, 141.69 ¿E) for the northern hemisphere and Kerguelen (49.35 ¿S, 70.24 ¿E) and Campbell Island (52.6 ¿S, 169.1 ¿E) for the southern hemisphere are used for analysis. It is found that there is a strong tendency for hemispheric similarities in the seasonal variations of oscillation amplitude, period, and probability of existence. The annual variations of the oscillation amplitude are modulated by the 12-month hemispheric and semiannual geomagnetic waves; the dominant period of oscillation is maximum in summer (50--51 hours) and minimum in winter (47--49 hours), and the probability of existence has a maximum in the summer half year and minimum in the winter half year. The seasonal similarities in the annual variations of the oscillation period and the probability of existence show the possible influence of the planetary 2-day wave in the middle neutral atmosphere on the electron density variations in the F region.¿ 1997 American Geophysical Union