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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 <I>X</I>=&minus;15.2 and &minus;12.7 <I>R_E</I> and P3 and P4 were at <I>X</I>=&minus;7.9 <I>R_E</I>. All four probes were at &minus;6.5&gt;<I>Y</I>&gt;&minus;7.5 <I>R_E</I> (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 (<I>B</I>₁) at velocities of 20–30 km/s. These are typical characteristics of current sheet flapping motion. The observed anti-correlation between ∂<I>B</I>₁/∂<I>t</I> and the Z-component of the bulk velocity make it possible to estimate a flapping amplitude of 1 to 3 <I>R_E</I>. The cross-tail scale wave-length was found to be about 5 <I>R_E</I>. Thus the flapping waves are steep tail-aligned structures with a lengthwise scale of &gt;10 <I>R_E</I>. 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.