Workplace behaviors and norms are gaining increased prominence in the literature on start-up enterprises. Research attention has focused on the critical role that these behaviors and norms play in influencing start-up effectiveness but our understanding of their role in influencing trust in a start-up environment is underdeveloped. The present article investigates the role that workplace social sexual behaviors play in shaping co-worker trust within start-ups. Using data from the GUESSS (2018) international study of entrepreneurial attitudes and experiences, we find that certain social sexual behaviors undermine trust, and related outcomes such as the willingness to delegate and the sense that co-workers are honest. In particular, experiencing inappropriate looks, flirtation, or sexual gossip predict lower levels of co-worker trust. Our findings also indicate that characteristics of the source of the behavior are important in terms of gender and hierarchical relationship. In our discussion section, we consider the mechanisms underlying these relationships and in particular how social sexual behavior may influence trustworthiness. Taken together, our results point to a significant efficiency cost to new enterprises that take a permissive view of social sexual behavior in the workplace.