This paper aims to contribute to the analysis of within industry inter-firm variety. The paper develops two themes: (i) the analysis of intra-industry heterogeneity, and (ii) the extent to which higher performance is associated with the capacity of firms to expand their knowledge base. The main contribution of the paper is empirical, based on a data source consisting of information on documents published in international scientific journals by Spanish pharmaceutical firms. The empirical results support the argument that the firm§s knowledge base is a main driver of persistent heterogeneity within industries. We find systematic variety in terms of how firms articulate their research activities, and positive correlation between firms? knowledge diversification and performance.