Alfalfa plants form bacteria-free nodules in response to a number of agents, including Rhizobium meliloti exo mutants, Agrobacterium tumefaciens transconjugants carrying cloned R. meliloti nodulation genes, and compounds that function as auxin transport inhibitors, N-( l-naphthy1)phthalamic acid or 2,3,5-triiodobenzoic acid. These bacteriafree nodules contain transcripts for the nodulins Nms30 and MsENOD2; transcripts for late nodulins like leghemoglobin are not detected. In situ hybridization studies demonstrated that ENOD2 transcripts were localized in parenchyma cells at the base and along the periphery of nitrogen-fixing alfalfa root nodules. The ENOD2 gene was also expressed in a tissue-specific manner in nodules elicited by N-( l-naphthy1)phthalamic acid and 2,3,5-triiodobenzoic acid. In bacteria-free nodules induced by R. meliloti exo mutants and A. tumefaciens transconjugants carrying either one or both R. meliloti symbiotic plasmids, ENODS transcripts were also detected but were usually localized to parenchyma cells at the base instead of along the periphery of the nodule. On the basis of the pattern of ENODS gene expression, we conclude that the developmental pathway of bacteria-free nodules, whether bacterially or chemically induced, is the same as that of nitrogen-fixing nodules, and, furthermore, that the auxin transport inhibitors in their action mimic some factor(s) that trigger nodule development.