Differential RNA display was used to analyze gene expression during the early steps of mycorrhiza development on Pisum sativum following inoculation with Glomus mosseae. Seven out of 118 differentially displayed cDNA fragments were subcloned and sequenced. One fragment corresponded to part of the fungal 25S ribosomal RNA gene and a second one showed similarity to a human Alu element. The others were derived from plant genes of unknown function. One of the fragments was used for the isolation of a full-length cDNA clone. It corresponded to a single-copy gene (psam1) which is induced during early symbiotic interactions, and codes for a putative transmembrane protein. Northern and RNA dot blot analyses revealed enhanced accumulation of psam1 RNA after inoculation with G. mosseae of wild-type pea and an isogenic mutant deficient for nodule development (Nod⁻, Myc⁺).