Petrographic analysis of potsherds from Dzibilchaltun and other Maya sites conclusively establishes the presence of volcanic ash temper in ceramics from northern Yucatan. The distribution of ash-tempered ceramics in time and space suggests import of ash in bulk from sources in highland Guatemala or E1 Salvador in exchange for salt. The homogeneous nature of the ash in northwestern Yucatan supports the idea that certain trading organizations enjoyed exclusive access to that region, while competing for markets in other lowland areas. -- AATA