Abstract Objectives: The purpose of this study was to examine in vitro the anti-bacterial efficacy of two different laser systems (CO(2) and diode) applied to Streptococcus sanguinis or Porphyromonas gingivalis cells in suspensions or adhered to zirconia or titanium dental implant materials, with two different surfaces each. Materials and methods: Bacteria were irradiated at two different power settings with either a CO(2) (lambda=10,600 nm) or a diode laser (lambda=810 nm). The lower mode is used clinically (for CO(2) 100 J/cm(2), diode 50 J/cm(2)) and the higher may alter the materials' surface (for CO(2) 1200 J/cm(2), diode 150 J/cm(2)). After irradiation, the number of viable bacteria was determined by culture. Results: Planktonic cells of both species were more resistant to the laser irradiations than bacteria that adhered to surfaces. Adhered P. gingivalis were effectively killed at both wavelengths lambda=10,600 and 810 nm even at the lower settings, independent of the material. S. sanguinis cells that adhered to either zirconia surface were effectively killed by the CO(2) laser at the lower setting of 100 J/cm(2). However, the higher settings of both lasers were needed to reduce S. sanguinis that adhered to titanium surfaces. The CO(2) laser at the lower setting and the diode laser at the higher setting effectively reduced the viability of S. sanguinis or P. gingivalis that adhered to zirconia surfaces. Conclusions: Under irradiation conditions known not to alter zirconia implant surfaces in vitro, both CO(2) laser (100 J/cm(2)) and the diode laser (150 J/cm(2)) effectively reduced the viability of adhered S. sanguinis or P. gingivalis.