n this paper, we present a new approach for indexing and discovering services in distributed systems. We propose to describe and index a service by properties (characteristics): its static properties (like its location, its provider name, etc.), its dynamic properties (behaviour) and its interface (parameters, methods, etc.). These three levels provide a complete vision about the service description. They can be used by clients to query and discover services. For this purpose, we propose a trader which relies on ontologies and knowledge representation to index and store the characteristics of services. The association of an ontology and knowledge representation provides a powerful combination that we will examine in the document. This development is compatible with the trader interface as defined by ODP and CORBA. Based on the description of the service behaviour and its interface, we propose in this paper, an approach which allows the client to compose and to combine services dynamically.