The altitude development of the 2-day wave in the ionospheric F region (from 170 to 230 km) has been investigated from the true height electron density profiles obtained at Observatori de l'Ebre station (40.8 ¿N, 0.5 ¿E). We found five time intervals of enhanced activity of the 2-day wave, bursts type, during 1995. The altitude variations of the 2-day wave period, amplitude, and phase are presented. The phase and group velocities of the wave are evaluated for each case. The observed changes in the electron density indicate three types of dominant vertical propagation of the 2-day wave in the F region: (1) upward propagation of the wave, (2) simultaneous 2-day oscillation of the electron density at all altitude range under study, and (3) downward propagation of the wave. The potential mechanisms proposed for explaining the vertical propagation of the 2-day wave in the ionospheric F region are discussed in order to evaluate the most likely mechanism. ¿ 1998 American Geophysical Union