Cenchrus ciliaris and Chrysopogon aucheri are important range-grasses found in Pakistan every where upto 1800 meters. Field and laboratory showed that beside toxic root exudates, water extracts continued substances which proved to be inhibitory not only to its own growth but also to other species used in the bioassays. The toxicity in both the cases increased with increasing concentration and soaking time. Cinhrus had more allopathic potentiality than Chrysopogon. The toxicity in both the cases was species slated. The possible effect of allelopathy of these grasses is discussed in relation to the vegetation dynamics.