This thesis is based on a case study of the design and use of management control systems (MCS) in a post-merger organisation. The study, which is motivated by the lack of literature on management control systems under mergers and acquisitions, is based on a Nigerian bank that has gone through a merger under the Central Bank of Nigeria's (CBN) directives to consolidate the banking sector. Data for the analysis was gathered from two sources. Firstly, semi-structured interviews were conducted with managers at various levels of the bank. Secondly, various internal and external documents were examined to corroborate the findings from the interviews. Theoretically speaking, the study drew on different strands of institutional theory to provide an understanding and interpretation of the results. First, new institutional sociology (NIS) provided an explanation of the impacts of external institutions on the merger as well as the design and use of post-merger controls. Old institutional economics (OIE) and power framework were also drawn on to explain the impact of internal institutions and power relations on the merger and the design and use of post-merger controls. The study found evidence of how various coercive, mimetic, and normative isomorphic forces as well as intra-organisational power relations contributed to the institutionalisation of controls as rules and routines in the post-merger organisation. The study contributes to knowledge by extending our understanding of the management controls literature on mergers and acquisitions.