Cassius Dio's fragmentary Roman History 15 contains an account of Archimedes’ role in defending Syracuse during the Roman siege of 213–212 b.c., incorporating a legendary tale about a solar reflector Archimedes constructed to burn Roman warships, and including details of his death when the city fell. The textual basis of this famous episode depends on two derivative twelfth-century works: Zonaras’ Epitome of Histories (9.4–5) and Tzetzes’ Chiliades (2.35). After clarifying the present state of enquiry, this paper introduces two new witnesses, (...) overlooked by editors of Dio and extensive scholarship on Archimedes, and assesses their value for reconstructing Dio's text. Comparative analysis of corresponding Dio-derived material in Tzetzes’ Carmina Iliaca and Hypomnema in S. Luciam, especially verbal correspondences with Zonaras’ Epitome, demonstrates that they are independent and, sometimes, superior witnesses to Dio's wording and content, reflecting Tzetzes’ selective use of the Roman History in different verse and prose compositions over several decades. The study considers editorial implications for this section of Dio's work and general characteristics of Tzetzes’ writings as repositories of testimonia and fragments. (shrink)