Social networks and entrepreneurship in biotechnology. The clustering of firms around regional centers of knowledge production. This paper addresses the clustering of new biotechnology firms around centres of knowledge production, by focusing on the role played by social networks in terms of access to scientific and technological knowledge. Our approach takes into consideration the impact of various forms of proximity – geographical, social, cognitive and organisational – upon the development of key relationships, as well as upon their use for knowledge acquisition. It enables us to assess the relative importance of local and distant knowledge sources, and to interpret the decisions taken by entrepreneurs with respect to network mobilisation. The empirical operationalisation of this approach is then discussed based on a suggested methodology that makes it possible to reconstruct social networks and to measure different types of proximity.