Aim: To investigate the bactericidal effect of gallium aluminium arsenide (GaAlAs) laser on photosensitized predominant periodontopathogenic anaerobic organisms. Materials and methods: The present prospective study was carried out among 50 patients of chronic periodontitis with pockets. 5 mm depth. For sample analysis of P. intermedia and P. gingivalis, a subgingival plaque was collected from periodontal pockets using sterile curettes under aseptic conditions. The sample was then immediately transported to microbiology laboratory using Transport media, Thioglycollate broth with Vitamin K and Hemin. After incubation period again the number of the viable bacterial count was carried out using a magnifying glass and expressed as CFU/mL, to determine the bactericidal effect of GaAlAs laser. A predetermined number of colonies (for P. intermedia 500 colonies were taken initially; for P. gingivalis 400 colonies were taken initially) were taken equally as control group and case group. The data was statistically analyzed using the SPSS statistical software version 16. Mann. Whitney Test was used for statistical analysis with p value less than 0.05 considered statically significant. Results: Study group shows a reduction in colony count of P. intermedia after being exposed to GaAlAs diode laser for 1 minute from 369–229, 134 and 41 which was statistically significant with p value <0.0001. Study group shows a decrease in colony count of P. gingivalis after being exposed to GaAlAs diode laser for 1 minute from 286.52–197, 94 and 39 which was statistically significant with p value <0.0001. Conclusion: The result of our study implies that for all tested bacterial strains, the effect of GaAlAs laser for 1 minute resulted in a significant reduction in the viable counts of photosensitized predominant periodontopathogenic anaerobic organisms. Photodynamic therapy (PDT) kills the bacteria and also leads to the detoxification of endotoxins. Further, it can thus be safely concluded that this technique is an alternative method adjunct to mechanical therapy. Clinical significance: The application of an alternative method to eradicate bacteria from periodontal pockets is desirable. One such approach is photodynamic therapy (PDT), i.e., laser therapy. Owing to the emergence of antibiotic resistance, PDT has to turn out to be a feasible alternative antibacterial therapy for biofilm-related diseases.