This paper proposes that diversification taxes firms’ existing organizational systems by altering routines, formal contract structures and strategies. I test the proposition that organizational adjustment costs associated with diversification erode incumbent competitive advantage, using novel microdata on taxicab firms from the Economic Census. The tests exploit exogenous local characteristics of taxi markets to identify the impact of diversification on firm organization and performance. Supporting the contention that diversification leads to organizational adjustments, the results show that diversifying firms are less likely to adopt computerized dispatching systems for their taxicabs and make significant changes in their formal contract structures governing asset ownership. Consistent with the theory, diversification is associated with falling taxi productivity. Comparing the productivity of diversified and focused start-ups and incumbent firms reveals that the organizational change component of diversification accounts for an 18% decrease in paid ride-miles per taxi. The results support the core contention of the paper that diversification taxes firms’ existing organizational capital.