Regional technology clusters are an important source of economic development, yet in biotechnology few successful clusters exist. Previous research links successful clusters to heightened innovation capacity achieved through the existence of social ties linking individuals across companies. Less understood are the mechanisms by which such networks emerge. The article uses social network analysis to examine the emergence of social networks linking senior managers employed in biotechnology firms in San Diego, California. Labor mobility within the region has forged a large network linking managers and firms, while ties linking managers of an early company, Hybritech, formed a network backbone anchoring growth in the region.