Molecular pathways governing the life cycle of arbuscular mycorrhizal (AM) fungi and their symbiotic interactions with root tissues are not yet fully understood. Most studies of fungal responses to host plants have targeted developmental stages before root contact (germinating spores), or after root colonization (intraradical mycelium). We are focusing on the early cell events of appressoria contact with the root surface which are essential to the successful outcome of the AM symbiosis. Recent monitoring of Glomus intraradices gene expression at this stage has revealed differential fungal responses to roots of host and non-host (Myc- mutants) M. truncatula (Seddas et al. submitted), suggesting a fine regulation of fungal genes by the host plant. Transcription factors are central regulators of gene expression which control many physiological and developmental processes but little is known of these elements in AM fungi. We have identified seven sequences in G. intraradices with similarity to genes encoding transcription factors in other fungi, by homology searches in databases. Transcript profiling of the corresponding genes in appressoria formed on roots of wild-type or Myc- mutants of M. truncatula points to their differential expression when symbiosis-related plant genes are inactivated, suggesting that the encoded transcription factors could be implicated in regulating early events leading to root colonization. Characterisation of the corresponding genes and analysis of transcription factor is on-going in order to obtain new insight into their role in symbiosis establishment.