The pathogenicity of three isolates of the genus Pythium (P.afertile , which was isolated from okra , hay isolate and cress isolate ) for many economical plants such as maize , sesame , cucumber and tomato were studied in artificially infested soils . These isolates were isolated from the roots of some infected plants and it's pathogenicity was determined according to it's ability to cause pre – and post emergence damping – off of seedlings. The pathogenicity tests showed that the P. afertile isolate recorded the maximum reduction in percentage of pre – emergence damping – off , where it's percent was 39.99% , followed by cress isolate as 32.08 % and hay isolate as 27.08% , whereas the cress isolate gave a significant reduction in percentage of post – emergence damping – off which was 27.35 % , followed by P. afertile and hay isolates which were 21.44 % and 16.35 % respectively .The antagonistic activity between the fungi that isolated from the rhizosphere of tomato and sesame plants and both Pythium's isolates (P. afertile and cress isolates) was studied on P.S.A. culture medium . The results of fungi isolated from the rhizosphere of tomato and sesame plants showed the presence of 13 different types of fungi in addition to Pythium's isolates which are belong to the following genera : Alternaria , Aspergillus , Cladosporium , Penicillium , Phytophthora , Rhizoctonia , Trichoderma , Verticillium and Fusarium . The microbial analysis showed the presence of all isolated fungi that mentioned above except the fungus Fusarium in the rhizosphere of tomato plant and both of the fungi A. wentii and Cladosporium in the rhizosphere of sesame plant. Results of this research demonstrated convergence in the response of antagonistic fungi against both Pythium isolates except the fungus Trichoderma , which was the most influential with inhibition rate reached to 66.37 % because of it's ability to mycoparasite pathogenic fungi .