In an earlier paper it was suggested that relationships between population density, migration, and the frequency of consanguineous marriage depended on variation in the social class composition of populations. For two valleys in the western Pyrenees, it was found that migration distances are shorter in areas with lower population density and that this could in part be attributed to the effects of social class independently of the occupations of the population. The present paper investigates further the relationships between migration, the (...) frequency of consanguineous marriage and inbreeding in the Roncal valley in Navarre, Spain, for the period 1877–1915. The Roncal valley contains seven villages, each a separate administrative unit. Almost all the total population of 3900 live in these nucleated villages and there are few isolated households. The area of the valley is 420 km2, and the population density 9·28 inhabitants/km2. (shrink)