We describe a pulsed gas valve which we have developed for use as a molecular beam source. In order to observe the performance of the pulsed beam source, we also have developed an ionization detector with a rise time of about 1 micros. The pulsed valve produces very intense supersonic molecular beam pulses of about 10 micros duration for light gases such as H2 and He, and of somewhat longer duration for heavier gases. As a new tool for the study of molecular collisions, the pulsed beam technique offers substantial advantages over the conventional continuous-beam method for experiments which are limited by the signal-to background ratio for scattered products.