Trust in others' honesty is a key component of the long-term performance of firms, industries, and even whole countries(1-4). However, in recent years, numerous scandals involving fraud have undermined confidence in the financial industry(5-7). Contemporary commentators have attributed these scandals to the financial sector's business culture(8-9), but no scientific evidence supports this claim. Here we show that employees of a large, international bank behave, on average, honestly in a control condition. However, when their professional identity as bank employees is rendered salient, a significant proportion of them become dishonest. This effect is specific to bank employees because control experiments with employees from other industries and with students show that they do not become more dishonest when their professional identity or bank-related items are rendered salient. Our results thus suggest that the prevailing business culture in the banking industry weakens and undermines the honesty norm, implying that measures to re-establish an honest culture are very important.