We have used a computer simulation to study gravity wave modulation of midlatitude sporadic E(Es) layers. It is shown that a horizontally stratified Es layer may be deformed by gravity waves and become a large-scale wavelike structure. Spatial resonance is not required for significant modulations to form. For a southward propagating gravity wave the south side sections of the wavelike deformed Es layer may overturn and appear as field-aligned features stretching about 10 km in altitude. The deformed Es layer drifts at a velocity larger than neutral wind velocity but smaller than the phase velocity of the gravity wave. The scale length of the wavelike Es layers is determined by the horizontal wavelength of the gravity wave. If there are two or more Es layers modulated simultaneously by a gravity wave, the height dependence of the amplitude of the wave must be taken into account. The Es layer at lower altitude is weakly disturbed if the gravity wave has only a small amplitude there. In contrast, the Es layer at higher altitude may be deeply modulated and appear as field aligned, since the amplitude of the gravity wave has increased greatly in comparison with the amplitude at lower altitude. The numerical results may be used to explain the midlatitude E region field-aligned irregularities observed by the Middle and Upper atmospheric (MU) radar. ¿ American Geophysical Union 1996