Based on the census data of 1970 and 1980 for each province of Thailand, the present study has revealed that between 1960-1980, a rapid fertility decline took place in 17 northern provinces and 26 central provinces of Thailand. A similar rapid fertility decline also took place starting in 1975 in the 16 northeast provinces and in the 14 southern provinces. Levels and trends of fertility found in this study suggest that most of the districts in the northern and the central regions reached the replacement level or below by 1985, and that similar levels will be attained by 1990 for almost every district in Thailand. This paper also stresses the impact of the rapid fertility decline on development in Thailand during the past 20 years, and its consequences. Lastly, the present study suggests that, for the future, the most important step is to understand well the consequences of this astonishing rapid fertility decline. This knowledge will be vital for the country and its people.