In this paper, the soybean 'early nodulin' clone pGmENOD40 is characterized. The GmENOD40 encoded protein does not contain methionine and does not show homology to proteins identified so far. In situ hybridizations showed that this gene has a complex expression pattern during development of determinate soybean nodules. At early stages of development transcription is induced in dividing root cortical cells, the nodule primordium and the pericycle of the root vascular bundle. In mature soybean nodules, the gene is expressed in the uninfected cells of the central tissue and in the pericycle of the nodule vascular bundles. Studies on nodules devoid of intracellular bacteria and infection threads, showed that the expression of the gene in the nodule primordium is induced in these empty nodules, while the induction of the GmENOD40 gene in the nodule vascular bundle requires the presence of intracellular bacteria or infection threads. A pea cDNA clone homologous to GmENOD40 was isolated to enable in situ hybridization studies on indeterminate nodules. The expression patterns in both determinate and indeterminate nodules suggests that the ENOD40 protein might have a transport function.