This paper reports on research carried out into structuring modular bills of material (BOM) in a company manufacturing heavy duty truck transmissions. The problems encountered in the application of the conventional option sensitivity approach to bill of material structuring on a sample of the company's products are summarized. The problems are related to dependence on part names and descriptions and the time involved in the task. This is followed by an account of the development of an improved technique (usage pattern analysis) for modularization. The technique is based on the concept that parts belonging to the same module must have identical patterns of usage. The use of the technique, in particular when computer-aided, reduces considerably the time needed for modularization.