The aim of this paper is to test the hypothesis that there can exist a well-defined quantitative relationship between the economic performance of an LDC economy and its speed of urbanization. The significance of economic motives for rural-urban migration in LDC's for individual migrants is well-established. The present analysis no longer focuses on the micro-economic determinants of mobility but on the possible existence of a stable functional relationship between short-term changes in the dynamics of the national economy and the transfer of population from the farm to the non-farm sector. It is shown that the annual net out-migration rate from the farm sector in Korea can be explained very well by short-term economic fluctuations. A distributed-lag model based on the growth rate of the non-farm sector and the farm terms-of-trade explains over 80 percent of the variance of the annual migration rate over the period 1955-1975. Alternative specifications of the model show that the Todaro hypothesis of migration which has improved the analytics of the decision to migrate by an individual at a single point in time is not particularly helpful in explaining the aggregate net rural-urban migration rate over time, at least in Korea.