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Biosynthesis of monodispersed nanoparticles, along with determination of potential responsible biomolecules, is the major bottleneck in the area of bionanotechnology research. The present study focuses on an ecofriendly, ambient temperature protocol for size controlled synthesis of gold nanoparticles, using the fungus Aspergillus terreus IF0. Gold nanoparticles were formed immediately, with the addition of chloroauric acid to the aqueous fungal extract. Synthesized nanoparticles were characterized by UV-Vis spectroscopy, TEM-EDX, and XRD analysis. Particle diameter and dispersity of nanoparticles were controlled by varying the pH of the fungal extract. At pH 10, the average size of the synthesized particles was in the range of 10-19 nm. Dialysis to obtain high and lowmolecular weight fraction followed by FTIR analysis revealed that biomolecules larger than 12 kDa and having -CH, - NH, and -SH functional groups were responsible for bioreduction and stabilization. In addition, the synthesized gold nanoparticles were found to be selectively bactericidal against the pathogenic gram negative bacteria, Escherichia coli. [ABSTRACT FROM AUTHOR]