Workers involved in a business merger often display strong ingroup/outgroup biases that can threaten the merger's success. Social identity theory helps to explain why and when such problems will occur. Using that theory, strong cohesion and successful performance were identified as two characteristics of a workgroup that should increase its resistance to a merger. An experiment involving mergers between small task groups was conducted to test this claim. Each group's cohesion and performance was used to predict its enthusiasm for a merger before it occurred, and any ingroup/outgroup biases that it displayed afterwards. Cohesion was unrelated to either of these measures, but as we predicted, more successful groups were less enthusiastic and displayed stronger biases. Relative rather than absolute success was an especially good predictor of merger resistance. The results were discussed within the context of social identity theory, which generated several suggestions for further research on business mergers.