We investigated the effects of fungicides on the common symbiosis arbuscular mycorrhiza (AM) in tomatoes plant. The infection of roots was delayed for three weeks by benomyl treatment and remained very low during the examination period. The intensity of mycorrhiza was also decreased (6.4% as compared to 44% in control roots) and remained more restricted than in the control. Arbuscules formed two weeks later, remained less developed and more restricted than in control roots. Captan inhibited the infection of roots and decreased the intensity of mycorrhiza (14 % as compared to 44 % in control root) similarly to benomyl. When the soil was treated with the two fungicides in combination, mycorrhiza did not appear at all on the roots for the 15 weeks examination period. The data demonstrate that fungicides used against different soil-borne pathogens of tomato plants have significant side-effects on AM in loamy soil, the extent of which may be dependent on the type of fungicide.