Objective (s): In recent years, the biosynthesis of gold nanoparticles has been the focus of interest because of their emerging application in a number of areas such as biomedicine. In the present study we report the extracellular biosynthesis of gold nanoparticles (AuNPs) by using a positive bacterium named Streptomyces fulvissimus isolate U from rice fields of Guilan Province, Iran.Materials and Methods: From over 20 Streptomyces isolates collected, isolate U showed high AuNPs biosynthesis activity. To determine its taxonomical identity, its morphology was characterized by scanning electron microscope and partial molecular analysis performed by PCR. In this regard, 16S rDNA of isolate U was amplified using universal bacterial primers FD1 and RP2. The PCR products were purified and sequenced. Sequence analysis of 16S rDNA was then conducted using NCBI BLAST method. In biosynthesis of AuNPs by this bacterium, the biomass of bacterium exposed to the HAuCl4 solution.Results: The nanoparticles obtained were characterized by UV-Visible spectroscopy, transmission electron microscopy (TEM) and Energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction spectroscopy (XRD) analyses. Our results indicated that Streptomyces fulvissimus isolate U bio-synthesizes extracellular AuNPs in the range of 20-50 nm.Conclusions: This technique of green synthesis of AuNPs by a microbial source may become a promising method because of its environmental safety. Its optimization may make it a potential procedure for industrial production of gold nanoparticles.