The aim of this project is to better understand biodiversity of arbuscular mycorrhiza (AM) fungi at the community and population levels. Populations of AM fungi are studied in different habitats at several scales under the impact of disturbance and fertilization (phosphorus or/and nitrogen). The influence of these factors was addressed in a field experiment. This study focuses on Rhizophagus irregularis, the widespread model species, previously named Glomus intraradices. Competition between strains of this species was assessed under controlled conditions in a growth chamber experiment using mitochondrial large subunit of ribosomal DNA as a specific marker for Real Time PCR.