A powerful argon ion laser was used to excite gaseous chlorine molecules resonantly from the X 1 ::E ground electronic state to the Aall excited electronic state. Three progressions of P; R doublets which correspond to the excitation from 1=37, 13 and 39 of v=O of the Xl~state to v'=22. 1'=38; v'=18, J'=12 and v'=16, JI=38 of the A'a state, respectively, and then decaying to various vibrational levels of the XI L state were observed. Of those, only the excitation from v=O, J=38 to v'=22. JI=38 transition has been reported before. Due to the extremely small transition probabilities, the spectral lines are much weaker than those of iodine and bromine. Their positions and their relative intensities were analyzed and compared with the result calculated from the known molecular constants and with the known Franck-Condon factors. Our data are consistent with the calculated results within the range of experimental error.The monochromatic radiation of a powerful laser is an ideal light source in the study of resonance fluorescence effect. Since 1967 many diatomic molecules, such as Nat, K I • Li l • BaO· and CUO,l} have been studied by this method. There are two advantages of using lasers in the fluorescence study: (1) Due to the strong intensity of the laser output, transitions with small transition probabilities and of low molecular concentrations can be studied; (2) Due to its monochromaticity, molecules are resonantly excited only to few excited states, therefore the spectra obtained are much simpler and easier to be analyzed than the spectra obtained from other excitation methods.The halogen gases are among the most interesting samples in laser fluorescence studies because not only they can be excited by light from the powerful argon ion laser, but also their transition probabilities are small. The fluorescence and the lifetime measurement of bromine and iodine gases excited by the argon ion laser light have been studied thoroughly.l.e) On the other hand, only one progression of four P, R doublets of respnance fluorescence lines of the chlorine gas has been observed using 4880A laser light excitation by Holzer, Murphy and Bernstein,» The reason is probably that, even excited .by strong laser light, the spectra of chlorine gas are still very weak. In fact, the transition probability between the Ala and the