Grid computing is emerging as a new infrastructure to provide collaborative and secure resource sharing over multiple geographically distributed organizations. In this scenario, resource discovery is a very important component, since it is responsible to retrieve information about the resources that compose the grid. Traditionally, the kind of data to be retrieved by resource discovery mechanisms is statically defined. In a highly heterogeneous and dynamic environment such as a grid, statically defined searches are usually inappropriate. In this paper, we propose and evaluate an extensible resource discovery mechanism for grid systems, where the basic resource information retrieval can be extended to include userdefined specific resource searches. Our experimental results show that the proposed mechanism is able to incorporate new searches to our grid resource discovery service in reasonable time.