Naturalism is one of the last white patches on the historical map of Spanish literature. Up to now it was mainly the reference to the idealistically oriented history of ideas in Spain, and notably krausismo, that discouraged an 'ordinance survey' in any greater depth. But in Spain as elsewhere, as this study is the first to show, the naturalist literary debates, naturalist poetics and literary works were marked by a system of reference that was receptive to positivist and scientific influences: krausopositivismo. This phenomenon here supplies the dispositif for a structured analysis of naturalist literature in Spain, which as an epoch displays a very different profile from its counterpart in France.