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Azospirillum brasilense is a nitrogen-fixing plant growth promoting rhizobacterium with potential as biofertiliser for leguminous and non-leguminous plants. Agriculturally important crops, especially sugar cane, have been often inoculated with A. brasilense producing increases in growth parameters and yield in different soils and climatic regions. Sugar cane inoculated with A. brasilense has been grown in intercropping with legumes like beans and soybean. In field experiments of intercropping of sugarcane inoculated with A. brasilense and soybean carried out in Cuban soils, an increase in most of the growth parameters of sugarcane have been observed, but it is not known the effects produced on the other intercropped plant. This work analyzes the alterations in plant and nodule development, nitrogenase activity and colonization of roots by Azospirillum, in soybean inoculated with A. brasilense 8-INICA and Bradyrhizobium japonicum USDA-110. A. brasilense inoculation did not produced changes in the height and biomass of aerial parts of the plant or in nitrogenase activity in nodules. However increases in root and nodule biomass were detected with its inoculation. A. brasilense colonized root surface by adhesion to mucilaginous material where bacteria proliferate and are able to form microcolonies inside. The break caused by the emergence of a secondary root allowed the bacterium to enter in the intercellular spaces of plant root parenchyma and between epidermal root cells, allowing soybean root colonization. These results suggested that A. brasilense 8-INICA can live as an endophyte in soybean roots and could be a suitable candidate like inoculant of leguminous with agricultural purposes. [ABSTRACT FROM AUTHOR]