Attitudes toward risk ináuence the decision to diversify among uncertain options. Yet, because in most situations the options are ambiguous, attitudes toward ambiguity may also play an important role. I conduct a laboratory experiment to investigate the e§ect of ambiguity on the decision to diversify. I Önd that diversiÖcation is more prevalent and more persistent under ambiguity than under risk. Moreover, excess diversiÖcation under ambiguity is driven by participants who stick with a status quo gamble when diversiÖcation among gambles is not feasible. This behavioral pattern cannot be accommodated by major theories of choice under ambiguity.