In spite of an increasing number of studies on ethical climate, little is known about the antecedents of ethical climate and the moderators of the relationship between ethical climate and work outcomes. The present study conducted firm-level analyses regarding the relationship between chief executive officer (CEO) ethical leadership and ethical climate, and the moderating effect of climate strength (i.e., agreement in climate perceptions) on the relationship between ethical climate and collective organizational citizenship behavior (OCB). Self-report data were collected from 223 CEOs and 6,021 employees in South Korea. The results supported all study hypotheses. As predicted, CEOs' self-rated ethical leadership was positively associated with employees' aggregated perceptions of the ethical climate of the firm. The relationship between ethical climate and firm-level collective OCB was moderated by climate strength. More specifically, the relationships between ethical climate and interpersonally directed collective OCB and between ethical climate and organizationally directed collective OCB were more pronounced when climate strength was high than when it was low. Theoretical and practical implications of these findings are addressed herein.