In cereals cultivated in Southern Chilean Andosols, arbuscular mycorrhizal (AM) fungi may play a main role for phosphorus (P) uptake. Because P acquisition at early growth stages is crucial for cereals, we investigated the development of indigenous AM during the first 45 days after planting of two wheat, barley and oats varieties in two typical Andosols of the region, under plastic house. Minimum temperatures were between - 5 degrees C and + 5 degrees C at night and maximum between 18 degrees C and 30 degrees C during day. The results showed that root biomass of all species increased in both soils until 30 days and remained constant thereafter until 45 days. The intensity of AM infection (root area and root biomass infected) was low at 15 days, increased slightly from 15 to 30 days and increased sharply and significantly from 30 to 45 days. Plant species and varieties differed in root biomass formation but not in frequency and intensity of infection with AM structures. Thus, those cereals species and varieties with more root production had higher total mycorrhizal root biomass, and those may potentially benefit more from AM. It is also concluded that during early growth stages cereals invest first into root development and then into AM fungal biomass.