In vitro antimicrobil activity was examined for the aqueous extract of Pleurotus ostreatus (Jacquie ex Fr.) Kummer in nutrient agar (NA). Mueller-Hinton (MH) and Mc Conkey agar (MC) medium following the filter paper disc and agar well diffusion methods. The microorganisms studied include human pathogenic forms of Klebsiella, Siayhylocoeus, Pseudonionas, Cilrobaeler, Proteus, Enterobacteriaceae, Salmonella sy. and E. coli. The paper disc study was performed against a standard tetracyctine (TE, 30 pg) disc. The result of growth inhibitory effect of the well surpassed that of the filter disc form of the test extract. The Proteus sp (diameter of growth inhibition zone, 28 mm) was found to be most vulnerable towards the extract in case of NA medium and the Klebsiella sp. the least (10 min). In case of MH agar, the test organisms varied in their extent of susceptibility towards the extract. The extract showed its best effect towards the pathogenic microorganisms used in case of Mc Contey agar medium. Here. the mostly affected organism was the Proteus sp. in both the agar well ( 19 mm) and the filter disc (16 min) methods and the least affected one was the E.coli (0 mm).