During the 1970s, China experienced one of the most dramatic fertility declines ever observed. Total fertility rates fell by more than 50%, from above 6 in 1970 to about 3 in 1980. This decline coincides with the ‘Later, Longer, Fewer’ campaign, China’s first fertility control program (and predecessor to the ‘One Child Policy’). Using novel data sources, we first study behavioral responses in fertility and marriage related to the campaign. Theory also predicts that fertility decline should strengthen incentives for sex selection among households that prefer sons – but previous research has not focused on sex selection prior to the One Child Policy (and the coincident introduction of ultrasound technology). In contrast to existing literature, we therefore also study and present new evidence on sex selection during China’s rapid fertility decline.