Although entrepreneurial effort is crucial for venture success and entrepreneurs frequently work in teams, we lack insights how effort develops in entrepreneurial teams. We propose an affiliation-based model of entrepreneurial effort under threat which predicts that entrepreneurs will reciprocate their teammates’ effort when confronted with a threat, i.e., low venture performance and high environmental hostility. We test our model in a sample of 103 entrepreneurs nested in 51 teams in a longitudinal setting capturing entrepreneurial effort over 26 weeks. While we did not find a direct relationship between teammates’ effort and an entrepreneur’s subsequent effort, we found support for the crucial role of threat in triggering the contagion of entrepreneurial effort. We discuss our contributions for research on entrepreneurial effort, social motivation, venture performance, and the firm environment.