Capping agents stabilise the size of the nanoparticles in the range of 1-10 nm. Microbial surfactants as capping agents are beneficial replacements for chemically synthesised ones because of lower toxicity. Rhamnolipids are surfactants produced by Pseudomonas aeruginosa, having high affinity for metal ions. In this study, the authors used rhamnolipids for capping ZnS nanoparticles. The capped particles were stabilised in aqueous environment and its characteristics were studied using Fourier transform infrared spectroscopy (FT-IR), small angle X-ray scattering (SAXS), high resolution transmission electron microscopy (HR-TEM) and ultraviolet-visible spectra, fluorescence spectra. The particle Bohr radius was found to be 4.5 nm both by SAXS and HRTEM, thus proving rhamnolipid to be an effective capping agent for the synthesis of uniform nanoparticles. SAXS study not only reveals the particle size and distribution but also its self-affined agglomeration behaviour. This work is a novel method for stabilising nanoparticles in aqueous condition using biosurfactant.