Studies reported antibacterial effect of photosensitizers combined with visible light. Objectives: To examine the effect of visible light irradiation alone at wavelength of 400 to 500 nm, which are used to photopolymerized composite resin materials, on viability of oral microorganism. Methods: Strains of Porphyromonas gingivalis, Fusobacterium nucleatum, Streptococcus mutans and Streptococcus faecalis in suspension or on agar plate were exposed to halogen light sources (power density of 260 and 520 mW/cm2) and plasma arc light source (1,040 mW/cm2) for up to 3 minutes. Percent of surviving bacteria was determined in relation to non-irradiated samples. For comparison, similar bacterial samples were exposed to near-infrared laser (diode, wavelength of 830 nm) with similar irradiation parameters. Results: A phototoxic effect of the blue light sources on P. gingivalis and F. nucleatum was obtained. This phenomenon was observed without exogenous photosensitizers and was irradiation dose dependent. Over 95 % of P. gingivalis cells in suspension and nearly 100 % on agar plate were killed by plasma arc irradiation, whereas effect of plasma arc on F. nucleatum was lower (70 % in suspension and 90 % on plate). Light irradiation generated by halogen had a lower effect on P. gingivalis viability than the plasma arc (70 % in suspension and on plate). When F. nucleatum was exposed to halogen light for 2 minutes virtually 100 % of cells on agar plate and 65 % in suspension were killed. No such effect was observed on the gram positive organisms tested, S. mutans and S. faecalis. The use of near-infrared laser irradiation had no effect on all bacteria tested. Conclusion: These results suggest that visible light sources such as halogen or plasma arc commonly used in restorative dentistry, have a phototoxic effect on periopatogens such as P. gingivalis and F. nucleatum.