In this investigation, the cyanobacterium Nostoc muscorum exhibited antagonistic activity against Gram-positive and Gram-negative bacteria and filamentous fungi. The results indicated that the active substance produced maximally after 12 days of incubation in shaken culture at 35°C, at pH 8.0 in BG-11 medium. The increase in nitrate concentration of the medium led to an increase in the antimicrobial production. Chloroform was the best solvent for extracting the active material. The antagonistic material was purified using thin layer chromatography. The compound showed maximum absorption at 240nm. Infrared (IR) and nuclear magnetic resonance (NMR) indicated presence of γOH, γCH aromatic, γCH aliphatic, γCN, γCO, γCC and CO. Mass spectroscopy indicated that its molecular weight is 279. The results also indicated that the compound is phenolic compound.