This paper examines how Spanish techno-scientific discourses and practices shaped metropolitan Spanish and colonial Guinean bodies and identities. It focuses on the range of technologies of biopower--from fingerprinting and blood testing to racial and geographic discourses--that constituted Guinean bodies in ambivalent ways during two periods: the first decades of the 20th century, and the post-Civil War period of the Francoist regime. In the first decades of the 20th century, blood tests were imposed on the local population as a legal requirement for obtaining identity cards in colonial Guinea; the identity cards offered them a severely restricted citizen status, especially if they were female. Indeed, the new blood testing technologies played a key role in efforts to control, reform and identify 'natives', less as subjects than as labouring bodies. During Franco's dictatorship, following the end of the Spanish Civil War (1939), the colonies became a space for the reconstruction of a unified Spanish national identity through two key strategies: 'detribalization' and 'hispanicization', which were carried out through a web of techno-scientific practices--in medicine and psychology as well as geography and anthropology--that included fingerprinting, blood testing, measurements of intelligence and racial discourses. Under the Franco regime, these practices not only justified violent, racist forms of exploitation, but were also used to stake a claim on Guinean colonial territories and bodies by emptying them of their existing identities and then reconstituting them under a single Spanish national identity.