The genus Bacillus is present everywhere in the environment, it has environmental, industrial, and medical signifi cance. This heterogeneous group of bacteria shares unique characteristics, one of these is the resistance to physical and chemical agents. Laser light is one of the most interesting physical agents with a wide range of applications in all scientifi c fi elds. It has the ability to alter bacterial growth by triggering inhibitory or stimulatory mechanisms. In this study, laser parameters were varied to evaluate the eff ect of each one on the viability of Bacillus species. CW or chopped (532 nm) laser light at diff erent irradiation periods were examined. Results showed that tested bacterial species’ response to green laser light varied. Bacterial viability was dropped with increasing chopping frequency and exposure time. In conclusion, high repetition rate and prolonged exposure time increase the lethal eff ects of this type of laser