Nearly all developed countries have experienced dramatic declines in the seasonality of demographic behavior over the last century. This article examines seasonal behavior in the population of Taiwan between 1906 and 1942, during Japanese colonial rule. First, we analyze seasonal demographic cycles and their ties to the cultural calendar and agriculture. Second, we compare seasonality in two regions (Xinzhu and Tainan) that experienced different agricultural development under the colonial regime. Findings demonstrate that demographic seasonality was less pronounced when urban development and colonial agricultural intensification produced more heterogeneous seasonal patterns of labor. We find that changes in agricultural organization, rather than purely a shift to nonagricultural production, may significantly reduce established seasonal timing of demographic behavior.