This paper analyses the impact of activist hedge funds (AHFs) on post-merger workforce downsizing and operating performance. AHFs have been widely criticized for achieving short-term gains at the expense of other stakeholders, such as employees. The results show that AHF ownership and presence in acquiring firms is a significant determinant of post-merger employment reductions. There is little evidence that these mergers and acquisitions have better operating performance relative to other takeovers. However, there is a negative effect of AHF ownership on labour productivity. Overall, the results are consistent with the view that AHF involvement in takeovers does not lead to sustained gains in performance. 1 In 2015, Trian owned a 7.2% stake in Pentair plc and urged the company to increase value by facilitating 'prudent industry consolidation'. In that year, Pentair acquired a privately held component manufacturer for $1.8 billion in cash. In 2017, Pentair sold a valves and controls business it had acquired a few years earlier for $3.2 billion and decided to split its largest businesses, electrical and water products, into two separate companies (The Deal Pipeline, 2017).