Metallic nanoparticles (NPs) have a high number of applications and today is possible to find them in many instruments such as medical or electronic devices or materials of new synthesis. In our laboratory we use bacteria to produce NPs using green technologies thus, employing less polluting methods and cheaper than the chemical synthesis. We have already produced selenium NPs, chemically pure and with size ranging from 50-150 nm by using the bacterium Azoarcus sp. CIB. In addition, we are studying the molecular mechanisms underlying both, toxic metal resistance and the production of NPs.