Findings from recent survey and excavation projects in Ejutla, Oaxaca, enable a broader examination of marine shell use and exchange in ancient Mesoamerica. A variety of shell ornaments were manufactured from Pacific Coast species at the Ejutla site during the Terminal Formative/Early Classic periods. Comparisons of the Ejutla mollusc assemblage with shell ornaments found at other highland Oaxaca sites indicate shifts in the nature of shell-ornament manufacture and exchange during the Formative and Early Classic periods. These changes, in conjunction with other findings, signal shifting relationships late in the Formative period among Monte Albán, the Ejutla Valley, and the Pacific Coast. The recognized shifts in highland-lowland Oaxaca relations generally coincide with an expanded importance of Pacific Coast shell at Teotihuacan and in the Lowland Maya region.