Locating and dating remains of pre-Aztec hydraulic agricultural systems in the Basin of Mexico with conventional reconnaissance and survey methods is problematic. Near-infrared aerial photographs have revealed buried linear features resembling irrigation canals in several areas including the Tlajinga section of the pre-Columbian city of Teotihuacán. Excavations confirmed that the features are prehispanic floodwater irrigation canals. The most recent ones date to the Late Aztec period; the earliest canals were probably constructed in Teotihuacán times although this cannot be demonstrated conclusively because of mixing of deposits. -- AATA