PIP: In France, the decline in marital fertility began in 1800 and preceded the marital decline in other European countries by about 70 years. The French decline is generally regarded as differing from the decline in other European countries only in timing, and the decline in all these countries is attributed to the innovative use of modern fertility control methods, which once introduced, were widely and rapidly accepted. An alternative explanation for the early decline in French fertility is presented. It is argued that the fertility decline in France between 1800 and 1980 reflects an adjustment process. It was suggested that in a peasant society, in contrast to an industrial society, there are only a limited number of available economic niches, and economic pressures will be intense if the population growth rate increases rapidly. In order to keep the population growth rate in balance with the economic resources, the net reproductive rate (NRR) must be kept near unity. In France, unity was maintained by keeping the marital fertility rate in balance with the nuptiality and mortality rates. Data on the population growth rates, mortality rates, marital fertility rates, NRRs, and on life expectancy is presented to support this argument. In France, between 1700-1800, the population increased by 35% and between 1800-1900 by 38%. This increase was exremely small compared to that which occurred in other European countries. For example, in England the corresponding increases were 71% and 252%. These differences in growth were the result of differences in the marital fertility patterns of the 2 countries. Before 1800, marital fertility in France was similar to that of other European countries, but after 1800 the French rate declined rapidly. By 1840 it was 2/3 of the 1800 level, and by 1900 it was 1/2 of the 1800 level. In other European countries marital fertility did not begin to decline until the 1870s. The decline in France differed not only in timing, but in form. When these declines were plotted on a line graph, the differences in form were immediately apparent. The French decline was choppy, and there is even an increase in marital fertility between 1850-1870. In contrast, the decline in other European countries was sharp, consistent, and smooth. During the 1700s in France there was a marked increase in the age at marriage and in the proportion of never married individuals. By the late 1700s a strain in the nuptiality pattern was evidenced by a marked increase in the illegitimacy rate and in the number of premarital pregnancies among brides. Subsequently, there was an increase in the nuptiality rate. In addition, life expectancy leaped from 28-39 years between 1790-1820. To compensate for both the increase in nuptiality and the decline in mortality, couples apparently limited births. These efforts to keep the marital fertility rate, the mortality rate, and the nuptiality rate in balance are evidenced in the NRR which varied from only 0.97-1.08 throughout the 1740-1880 period. The adjustment explanation is supported by the fact that while the marital fertility rate varied between 1740-1880, the NRR rate; remained stable. A subsequent paper will analyze regional differences. In using a equilbrium model, it is necessary to determine if the unity observed in the national NRR is a statistical artifact, resulting from large regional differences canceling each other out at the national level.