Measurements of time-resolved photobleaching and nanosecond fluorescence decay from microscopic samples of methanogenic bacteria are reported. From cultures of Methanobacterium thermoautotrophicum and Methanosarcina barkeri, decay times of 1 ns and 3 ns were obtained for the specific coenzymes F420 and 7-methylpterin, respectively. In contrast to methylpterin, the fluorescence of F420 was bleached selectively, with a time constant of about 160 s, at an irradiation power density of 5 mW mm-2. Similar time constants were found for samples of sewage sludge containing methanogenic bacteria. Active and inactive bacterial cells could be differentiated by following the course of photobleaching.