Stimulated emission and lasing have been demonstrated for chlorophyll a , chlorophyll b , and bacteriochlorophyll a as well as for the Mg-free chlorophyll derivatives pheophytin a and methyl pheophorbide a . Population inversion was obtained by optically pumping with an N 2 laser. For the three chlorophylls, lasing was observed for only the (0,0) vibronic region of the S 1 → S 0 fluorescence band. In contrast, lasing was observed for both the (0,0) and (0,1) vibronic regions of the S 1 → S 0 fluorescence band of pheophytin a and methyl pheophorbide a . Differences between the lasing behavior of the chlorophylls and the Mg-free derivatives probably reflect differences in aggregation behavior. Laser pulses of duration shorter than the fluorescence lifetimes were generated. This result suggests that stimulated emission must be considered in the interpretation of fluorescence lifetime data for in vitro and in vivo chlorophyll-containing systems.