Mycorrhizal colonization was only found in the treatments with AMF inoculation (MR treatment and BMR treatment). Colo- nized roots comprised 8–26% of the total root length (Table 1). In the MR treatments (50 + 50 and 100 + 100 mg kg−1 ), no significant (p > 0.05) adverse effects of PAHs on the mycorrhiza infection rates were observed. At the highest level of PAH (200 + 200 mg kg−1 ), the mycorrhiza infection rates were significantly lower (p < 0.05) than that of unspiked control (at PAH level of 0 mg kg−1 ). Inocu- lation with PAH-degrading bacteria Acinetobacter sp. significantly (p < 0.05) increased the mycorrhiza infection rates of ryegrass roots in unspiked soil, but did not significantly (p>0.05) increase the mycorrhiza infection rates in soil spiked with PAHs. The inoculation of AMF significantly stimulated the growth of ryegrass while the biomass of plant was significantly decreased due to the presence of PAHs (Table 2). In general, the shoot and root biomass of ryegrass in all treatments decreased with increasing of PAHs concentration