A novel scheme of coUinear fast beam laser spectroscopy for the detection of the long lived rare isotope *^Kr by observing the optical hyperfme structure spectrum is presented. The technique utilizes cascade two-step excitation to pump metastable krypton atoms to a high-lying Rydberg level. The present work on krypton was motivated by the fact that *̂ Kr is a major tracer gas for exploring the reservoir structure of large oil fields. *^Kr detection in ambient air is also of importance for monitoring nuclear activities on a world wide scale. The technique has been successfully applied to stable krypton isotopes and to^^Kr. The selectivity is at the one part in lO'" level and the sensitivity at a few hundred ions/s.