This article traces the evolution of the idea of degeneration in urban Britain between the early nineteenth and early twentieth centuries. Rejecting approaches that reduce this richly eclectic, though savagely negative, world-view to a random bundle of prejudices underpinning the emergence of the ‘science’ of eugenics, the article focuses on distinctive environmental, medical and anti-urban determinants. Strong emphasis is also placed on shifting interactions between moral and medico-environmental values and prescriptions which served as legitimation for the racially inflected view that residual elements of the inner city working class might soon be doomed to physiological and hereditary extinction.