In this paper, the regional differences in fertility in Spain are analyzed statistically in relation to the level of urbanization and the socioeconomic composition of the population, with special attention to the baby-boom period around the 1960s. The main sources of data are the Censo de Población and the Movimiento Natural de la Población Española, published by the Instituto Nacional de Estadística. First, the 50 provinces of Spain were classified into four groups based on the evolution of the Index of General Fertility from 1887 to 1991, an indicator of period fertility extensively used in the European Fertility Project of Princeton University, This classification has been found useful in determining the process of fertility decline in which the low fertility area, initially limited to the northeast part of the country, was extended from the northern coast to the vast central region, and then to the south. Second, the complete cohort fertility of the population born in 1921-1925 was estimated for the provinces, the urban and rural areas of each province, and the 81 principal cities. The indicator used for this estimation was the mean number of children ever born to women aged between 45 and 49 years in 1970. Variance analysis applied to the disparities of cohort fertility between the provinces and those detected between the urban and rural areas shows that the variance, although statistically significant in both senses, is much more important between provinces. Although cohort fertility is commonly lower in urban than in rural areas, those cities that experienced a high rate of in-migration show higher cohort fertility than the rural hinterlands. This fact, along with the disparate values of period fertility by geographic origin among women living in Barcelona, suggests that the in-migrants contribute to raising the fertility by the fertility behavior of their native site. The cohort fertility in the principal cities has no significant association with the educational level or with occupational structure, except in the macro-regions around Barcelona and Madrid, where they show a loose association. This association, however, seems spurious, given the almost absolute absence of this kind of relation in the other parts of the country. It is more reasonable to suppose that the fertility in these two macro-regions is more directly related to the rate of in-migration, rather than to the two socioeconomic variables considered here. The results of this analysis suggest that, although urbanization has some degree of influence on the fertility, regional differences in fertility in Spain are difficult to explain exclusively by the level of urbanization and the socioeconomic composition of the population. Further research is required to determine the cultural and regional variables with special relevance to fertility behavior and to analyze the impact of migration on the fertility of migrants.