Abstract: Femtosecond laser ablation of solids usually carried out in vacuum or liquid environments, offers a novel and reliable method for the production of nanoparticles of different materials. In this work we have investigated the production of nanoparticles by femtosecond laser ablation (775 nm, 180 fs, 1 kHz, fluence: 0.05-1.6 J cm-2) of the target material in air. We focus mainly on silicon nanoparticles and use Atomic Force Microscopy, Transmission Electron Microscopy and Energy Dispersive X-ray Analysis. Si nanoparticles produced in air are single crystals with an average height distribution of 40-50 nm in areas where the debris is in the form of a continuous deposit, and 9-11 nm in areas where the debris is in the form of isolated particles. This is comparable to the average size distribu-tion of 6-12 nm of particles produced previously in vacuum. The nanoparticles have disk shape with long to short axis length ratio in the region 7-10 which is larger than the eccentricity of 1.4-6.6 found previously for silicon nanoparticles produced in vacuum.