Fertility has decreased very fast in some countries in Asia from the late 1970s, particularly in Indonesia, Viet Nam and above all in Bangladesh, with a fertility rate in Viet Nam that went down from 5.8 children per woman in 1970-1974 to 3.3 in 1991-1995. Catherine SCORNET here examines the circumstances of the transition in the Red River basin. The point she makes is that a forceful nationally defined demographic policy should necessarily take the local social and economic context into account if it is to be efficient. In this particular case the background is characterized by very high demographic density, with over 1,000 inhab. per sq. km. The fertility behaviour of households then depends both on traditional cultural values, economic opportunities other than agriculture and the precise modes of enforcement of the family planning programme.