Abstract The biosynthesis of nanoparticles is considered to be a novel, effective and eco-friendly procedure. In this paper, Magnetospirillum gryphiswaldense MSR-1 was successfully used to reduce gold ions to a zero-valent metal in a water environment, and more importantly, to accumulate them into spherical nanoparticles on the cell surface through biosorption. Transmission electron microscopy (TEM) was used to examine nanoparticle characteristics. The size range of the nano-gold made by MSR-1 was from 10 to 40 nm. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses of the gold nanoparticles confirmed the formation of metallic gold. The pH value of the biosynthesis solution and the initial metal concentration were examined; both of these important parameters were isolated and screened to determine their role in controlling particle size. The results demonstrated that lower pH values and initial concentrations of Au(III) were individually responsible for reductions in nano-gold particle size.