The production of negative ion beams for use in neutral beam injection heating has become an important area of research in recent years. This paper discusses the negative ion densities measured in a pulsed multicusp volume ion source using photodetachment diagnostic technique. A pulse modulated negative ion source is being used as an alternative to the tandem source and an increase in negative ion extracted current has previously been observed by Hopkins and Mellon. Work with photodetachment quoted in this paper shows an increase in negative ion density during the post discharge similar to previous results obtained using an accelerator to extract the negative ions.