Current historiography has considered eugenics to be an emanation from state structures or a movement which sought to appeal to the state in order to implement eugenic reform. This paper examines the limitations of that view and argues that it is necessary to expand our horizons to consider particularly working-class eugenics movements that were based on the dissemination of knowledge about sex and which did not aspire to positions of political power. The paper argues that anarchism, with its contradictory practice (...) afforded by the convulsive social situation of the Civil War in Spain, allows us to assess critically the parameters of the social action of eugenics, its many alliances, and its struggle for existence in changing political circumstances not of its own making. (shrink)