ABSTRACTInoculum potential ofGlomus mosseaeandGlomus etunicatumwas reduced significantly in pasteurized soil amended with a sieved suspension from non-sterile soil. Regardless of mycorrhizal fungus inoculum dosage, dry weight and root colonization of plants grown in pasteurized soil with non-sterile soil sievings never equalled those of plants grown in pasteurized soil alone. However, plants exposed to non-sterile soil were more colonized and grew faster than plants in pasteurized soil alone. Only after 13 wk were plants in pasteurized soil significantly more colonized, and after 16 wk larger in dry weight than their counterparts in non-sterile soil or pasteurized soil with sievings. Apparently, loss of inoculum potential alone cannot fully explain the suppression of plant growth observed in non-sterile soil. The suppressive effects of non-sterile soil may be best related to microbial mediation of nutrient availability in soil.