The debris produced by femtosecond laser ablation (180 fs, 775 nm, 1 Hz) of Si in ambient air is deposited around the ablated craters in a circular zone with diameters between ~40 and 300 μm for laser fluences (F) in the region F=0.2–8 J/cm2. The debris consists of nanoparticles. The mean height of the nanoparticles increases with laser fluence (from ~70 to 500 nm for fluences in the range F=0.25–4.38 J/cm2) but at high fluences (F=8 /cm2) becomes equal to ~170 nm. The average horizontal dimension of the nanoparticles increases with laser fluence. Their average vertical imension increases in proportion to their average horizontal dimension, but at high fluences becomes much smaller than their corresponding average horizontal dimension. The nanoparticles were found to be single crystals with d spacing of 1.71±0.08 Å.