The article deals with the problem of locating the exchange market at the Roman forum. The market existed there in the late republican and early imperial period. It is known that it was located at the fence of the temple of Castor not far from Puteal Libonis and the arch of Ianus medius. The location of the fi rst two is fi rmly established, but there are mutually exclusive hypotheses as to the Ianus medius’ location in the works of some researchers. In the present article on the basis of source analysis the exact place of Ianus medius and the exchange market is fi nally detected.