A 7.8 A, 50 keV H−ion beam was produced by a cesium seeded volume negative ion source. The source consists of a 25 cm×46 cm rectangular multicusp plasma generator and a 14 cm×36 cm multiaperture extractor. Without cesium, the source produced 3.4 A, 75 keV H−ion beams. By seeding a small amount of cesium, we observed a big enhancement of H−production efficiency by a factor of four, and a reduction of optimum operating pressure. Extracted electron current decreased to almost zero when we biased the plasma grid positive with respect to the anode. The effect lasted for more than a week once the cesium was injected for several seconds at an oven temperature of 280–300 °C.