A low-cost green and reproducible microbes (Lactobacillus sp. and Sachharomyces cerevisiae) mediated biosynthesis of US nanoparticles is reported. The synthesis is performed akin to room temperature in the laboratory ambience. X-ray and transmission electron microscopy analyses are performed to ascertain the formation of US nanoparticles. Individual nanoparticles as well as a few aggregate having the size of 2.5-5.5 nm are found. UV-vis spectroscopy study revealed the surface plasmon resonance at 393 and 369 nm respectively for Lactobacillus and yeast assisted synthesis of CdS nanoparticles. The absorbance spectra were used to estimate the values of optical band gap and particle size of US nanoparticles. A possible involved mechanism for the biosynthesis of US nanoparticles has also been proposed. (C) 2009 Elsevier Inc. All rights reserved.