An 18-megavolt, 1 megajoule Marx generator has been constructed and tested to 11 MV as the primary energy store of the Hermes II flash x-ray machine. A geometrical arrangement for the capacitors that takes advantage of the stray capacities to provide a wide triggering range and fast Marx erection time was developed from model and circuit studies. The design parameters of the Marx were checked by constructing and testing a 4 MV, 100 kJ generator using components proposed for the 18 MV system. Spark gaps were developed specifically for the generator and have operated successfully for over 50,000 gap firings.