In the use of spark gaps as switching devices, it is desirable to maximize the power delivered to the load and to minimize the power deposited in the switch; that is, it is desirable for the resistance of the switch to be negligible as compared to the load. The hydrodynamic time scale for expansion of the arc in a spark gap and hence for the reduction in its resistance to a small value is tens to hundreds of nanoseconds. Therefore, with current pulses of duration of a few hundred nanoseconds or less, the resistance of the spark gap may be a significant fraction of that of the load. In this paper, we report on measurements that determine the resistance of the arc in a fully diagnosed laser‐triggered spark gap. The spark gap switches a 100‐ns, 1.5‐&OHgr; waterline into a 1.5‐&OHgr; load resistor. A capacitive voltage divider housed within the switch enclosure measures the voltage drop across the switch, a current‐viewing resistor measures the current, and an interferometer measures the diameter of the plasma column, a value required to calculate its inductance. The resistance of the arc is found to remain in excess of 0.1–0.2 &OHgr; for the duration of the current pulse for a variety of switch gas mixtures. The resistance decreases with increasing charging voltage on the waterline at the time of triggering and decreases with decreasing average molecular weight of the gas mixture in which the arc is sustained.