Herodotus’ narrative of the testing of various Greek oracles by King Croesus of Lydia (1.46–54) has long been viewed with justifiable scepticism. A newly published verse dedication from the sanctuary of Apollo Ismenios at Thebes (Papazarkadas 2014: 233–48) sheds welcome light on Herodotus’ sources for this part of his Croesus-narrative. Herodotus’ account of Croesus’ testing of the oracle of Amphiaraus at Thebes appears to have been an imaginative extrapolation from the text of this inscription. But there is good reason to believe that Herodotus significantly misinterpreted the historical context and significance of the epigraphic text he had before him; in particular, the real author of the dedication is unlikely to have been King Croesus of Lydia, and may instead have been an Athenian aristocrat of the Alcmaeonid family. The new inscription from Thebes sheds light both on Herodotus’ use of documentary evidence and on the creative misreading of early epigraphic texts by Theban sanctuary personnel in the mid fifth century BC.