Pietro Metastasio’s opera Catone in Utica (Rome, 1727) represents ancient Roman imperial politics through the recurring trope of ‘enslavement’. Reading Catone alongside Metastasio’s sources, from Lucan to Addison, reveals how the poet’s de-particularizing representational code converted historical modes of racialization into a generalizing Cartesian moral framework, and thereby demonstrates how the continuing influence of post-Enlightenment constructs of biological race has obscured the multiplicity of racialisms in earlier contexts. Turning from a physiological episteme to an earlier, ‘unassimilated space’ limned by poetics, sentimentality, and song, this article takes Metastasian opera seria as a window onto historically contingent conceptions of racialized difference.