Inefficient implementation of global namespaces, mesage passing, and thread scheduling on stock multicomptuers have prevered concurrent object-oriented programming (COOP) languages from gaining widespread accptance. Recognizing that the architectures of stock multicomptuers impose a hierarcy of costs for these operations, we have described a runtime system whtich provides differrent versions of earch primitive, exposing performance distinciation for optimiation. We confirm the advantages of a cost-hierarchy based runtime sysem organizatin by showing a variation of two orders of magnitude in version costs for a CM5 implementation. Frequency measurements based ond COOP application program demonstrate that a 39% invocation cost reduction is feasible by simply selecting cheaper versions of runtime operations.