Morally, certainly yes, that’s why they’re the CEO. Their only defence against responsibility is incapacity to fulfil their duties, in which case they should resign immediately.
This isn't just a theory. On of my companies competitors (I do not know which, I was just given it as an example) was caught bribing officials in China. After investigation it was decided that it was a rouge manager and not the CEO: they has a policy of not bribing, everyone was trained in recognizing how something (even a meal) could be seen as a bribe, and when other unethical action had been taken in the past the company took action to stop it and treated the whistle-blowers well. The CEO got off because he did everything in his power to ensure this wouldn't happen.