Pythium oligandrum is one of the best known fungi in biological control of many fungal plant diseases. The objective of this study was conducted to highlight the possibility of using cultural filtrate of this fungus in inhibiting the growth of reproducting units of the pathogenic Pythium aphanidermatum and P. diclinum. Two isolates of each fungus were studied. Ability of P. oligandrum to grow and develop on V-8 liquid medium was tested in order to study its effect on mycelium growth,  zoospore and oospore production of each of the pathogenic pythia of P. aphanidermatum and P. diclinum. Cultural filtrate of P. oligandrum (quarter and half concentration) significantly inhibited mycelial growth and the production of zoospores and oospores of the pathogenic tested fungi. The rate of inhibition in all treatments reached more than 90% in case of the use of half concentration of the cultural filtrate of P. oligandrum. The study will provide information for the use of cultural filtrate of potential selected isolates of  P. oligandrum in biological control of some fungal diseases especially in aquaculture. The outcomes of this research can be easily applied in aquatic farming but need further work on different types of pathogenic fungi.