The Potential of Biological Agents to Inhibit Growth of Phytium sp. In Vitro. The study aimed at testing the potential of some antagonistic fungi isolated from durian seedlings media to inhibit growth of Phytium sp. Research was done at the Central Laboratory of Tropical Fruit Research Solok in July-September 2009 by using a complete randomized design with 5 treatments and 4 replications. Tests was conducted by dual culture method on PDA. The results showed that Gliocladium sp., Trichoderma sp.a, Trichoderma sp.b, Aspergilus sp., and Penicillium sp. can inhibit growth of Phytium sp., with growth inhibition of 50, 49.5, 47, 48, and 38.3% respectively. Inhibition mecanism of Gliocladium sp., and Trichoderma sp. were competition, antibiosis, lisis, and parasitism, while Penicillium sp. was antibiosis. Gliocladium sp., Trichoderma sp.a, Trichoderma sp.b, Aspergilus sp., and Penicillium sp. can be used as biological agents to control pathogenic fungi Phytium sp.