Culture filtrate of the blue-green alga Chroococcus minutus isolated from Wadi-Sannur, Egypt, was screened for its antimicrobial activity against 5 Gram positive and 3 Gram negative bacterial species, unicellular fungus Candida albicans and 9 filamentous fungal species. The filtrate exhibited an inhibitory activity against all bacterial species under investigation except Staphylococcus aureus, has a selective inhibitory effect against the fungal tested organisms. Further studieswere conducted to optimize the growth conditions and to enhance the productivity of C. minutus. The data revealed that pH 8, light/dark period of 16/8 h and light intensity of 2000 Lux were the optimum growth conditions for enhancing the cyanobacterium productivity. Moreover, the cyanobacterial culture filtrate was tested as a biocontrol agent against the phytopathogen Pythium sp < em>. which parasite on the tomato plant. Significant stimulating effects were produced on the tomato seeds which previously soaked in C. minutus culture filtrate, in retarding the Pythium sp. symptoms. This was obvious such as increasing seed germination percentages, survival percentages and growth parameters such as increasing fresh weight shoot and root length and chlorophyll contents.