Economic crises in the late Roman Republic and the associated problem of private debt are well known to scholars. But research has neglected a fundamentally important theory in modern economics that elucidates these issues: the theory of 'debt deflation'. This paper uses the debt deflation theory to examine the major credit and debt crises of the late Republic. We provide a summary of the modern theory of debt deflation and its relevance to the Roman economy in the 1st century BC, and then reanalyse the crises of 89-86 and 49-45 BC. It is concluded that these were 'debt deflationary' phenomena and that the modern theory sheds new light on this aspect of the late Republican economy.