Financial markets have witnessed a dramatic shift in financial flows in recent years from Active fund management where professional investors attempt to beat the market (generate ‘alpha’) to Passive investment where portfolios are assembled that follow existing market indicators (track ‘beta’). This transition has important implications for both corporate governance and wider society, with potentially significant distributive effects. Passive investing is predicated upon different bodies of knowledge and is suggestive of an epistemic shift of sorts in financial markets. In such circumstances, field theory suggests that incumbent groups like Active players will try to adapt to the new rules of the investment game. However, drawing from an empirical study which explores the views of the Active investment community in both the United Kingdom and the United States, we document significant defensiveness vis-à-vis the rise of Passive investing. Whereas behavioural approaches might explain this defensiveness in terms of irrationality, the conceptual approach advanced here instead emphasizes the epistemic opportunism (convoluted and self-serving attempts to demonstrate superior knowledge) that communities strategically engage in to justify their position. As such, we conclude that financial markets should be understood as constituted by slowly evolving communities of practice whose habits, routines and ways of knowing can be difficult to shift, even when faced with overwhelming evidence that what they are doing doesn’t work most of the time.