The support of the superclass-subclass concept in object-oriented databases (OODB) makes in instance of a subclass also an instance of its superclass. as a result, the acces scope of a query against a class in general includes the access scope of all its subclass, unless specified otherwise. to support the superclass, subclass relationship efficiently, the index must achieve two objectives. frist, the index must support efficient retrieval of instances from a single class. second, it must also suport efficient retrieval of instances from classes in a hierarchy of classes. in this paper, we propose a new index called the H-tree that supports efficient retrieval of instances of a single class as well as retrieval of instances of a class and and its subclasses. the unique feature of H-trees is that they capture the superclass-subclass relationship. a performance analysis is conducted and both exprimental and analytical results indicate that the H-tree is an efficient indexing structure for OODB.