The development of arbuscular mycorrhiza between obligate symbiotic fungi and host roots must involve activation of symbiosis-related genes. Using a new powerful method, called differential display reverse transcription polymerase chain reaction (DDRT-PCR) developed for eukaryotic cells, we have started to study early molecular events during mycorrhiza formation by pea roots and Glomus mosseae. (DDRT-PCR)-cDNA fragments corresponding to genes that are specifically enhanced or repressed during the first steps of the symbiosis are being isolated.