Abstract: An inertially operated valve for a fluid conduit which upon occuring a jarring force, such as an earthquake, the valve is moved to a shut-off position. The shut-off position occurs when a plug is caused to be jarred free from a holding position and then fall by gravity to block the flow of fluid through the valve. The holding of the plug in the retracted position, not blocking flow of fluid, is accomplished through the use of a magnet. A separate magnet is also utilized to hold the plug in the extended/blocking position. A resetting device is to be manually operated to move the plug from the extended/blocking position to the retracted position.