It can be difficult to visualize the shape and disposition of components in biological material from serial sections. They can be reconstructed by tracing outlines from sections onto an acetate sheet and stacking the sheets, but this method provides only a static model in which the elements cannot be manipulated or accurately measured. A more versatile method uses an interactive computer program. Data are entered via a digitizing tablet and the model may be displayed in any desired orientation and with any combination of elements presented. In this two-part paper we describe this technique and explain how it was used to elucidate the cellular composition of part of the feeding apparatus of a plant-infesting nematode. The oesophageal metacorpus of Aphelenchoides blastophthorus was found to consist of 43 cell bodies: 24 muscle, six epithelial and 13 nerve cells. The wide biological applicability of the technique is discussed and, in particular, its use in studying taxonomy and phylogeny of nematodes.