Phaseolus vulgaris (common bean) can be nodulated by several Rhizobium species. Among them, Rhizobium tropici has a relatively broad host range, as it is able to infect beans, Leucaena trees, and several other legumes. This work describes the isolation and the characterization of extracellular factors (Nod factors) whose production from R. tropici was triggered by the transcriptional activation of its nod genes. These factors consist of a chitopentaose backbone in which the N-acetyl group of the nonreducing end glucosaminyl residue is replaced by an N-methyl-N-vaccenoyl one. Some of these molecules are sulfated on position 6 of the terminal reducing glucosamine.