A sequence of magnetic field oscillations with an amplitude of up to 30 nT and a time scale of 30 min was detected by four of the five THEMIS spacecraft in the magnetotail plasma sheet. The probes P1 and P2 were at X=-15.2 and -12.7 R(E) and P3 and P4 were at X=-7.9R(E). All four probes were at -6.5 > Y >-7.5R(E) (major conjunction). Multi-point timing analysis of the magnetic field variations shows that fronts of the oscillations propagated flankward (dawnward and Earthward) nearly perpendicular to the direction of the magnetic maximum variation (B(1)) at velocities of 20-30 km/s. These are typical characteristics of current sheet flapping motion. The observed anti-correlation between partial derivative B(1)/partial derivative t and the Z-component of the bulk velocity make it possible to estimate a flapping amplitude of 1 to 3 R(E). The cross-tail scale wave-length was found to be about 5 R(E). Thus the flapping waves are steep tail-aligned structures with a lengthwise scale of > 10 R(E). The intermittent plasma motion with the cross-tail velocity component changing its sign, observed during flapping, indicates that the flapping waves were propagating through the ambient plasma. Simultaneous observations of the magnetic field variations by THEMIS ground-based magnetometers show that the flapping oscillations were observed during the growth phase of a substorm.