Nano-size particles of Ag were prepared by laser ablation in water with femtosecond laser pulses at 800 nm. The formation efficiency and the size of colloidal particles were compared with those of colloidal particles prepared by nanosecond laser pulses. The formation efficiency for femtosecond pulses was significantly lower than that for nanosecond pulses. The size of colloids prepared by femtosecond pulses were less dispersed than that of colloids prepared by nanosecond pulses. The shape of the craters on the targets suggested that ablation process was different between femtosecond and nanosecond ablation. In addition, it was found that the ablation efficiency for femtosecond ablation in water was lower than that in air, while that for nanosecond pulses was similar in both water and air. (C) 2002 Elsevier Science B.V. All rights reserved.