The electrohydrodynamic (EHD) instability in planar geometry was observed using gallium as the working fluid. Electric fields in the range of 0.12–0.26 MV/cm were applied in pulse durations up to 14.4 ms. The instability of the gallium surface was observed using a box camera and pulsed backlighting. The measured wavelengths of the instability ranged from 1.2 to 17.0 mm, consistent with theoretical predictions provided a field enhancement factor of 1.5 is assumed on the meniscus of the gallium. A transverse magnetic field of about 0.6 T was found to completely dampen the instability along the magnetic field at an applied electric field of 0.2 MV/cm; the transverse direction was unaffected. This result puts limits on the usefulness of an EHD ion source in magnetically insulated diodes for inertial confinement fusion studies. ©1995 American Institute of Physics.